Jun - American Radio History

Transcript

BENTLEY ACOUSTIC CORPORATION LTD. 38 CHALCOT ROAD, CHALK FARM,LONDON, N.W.1 THE VALVE SPECIALISTS Telephone 01-722 9090 SAVE POSTAL COSTS! CASH AND CARRY BY CALLERS WELCOME 6/4'6 30E1.1114'6 A.2131 19 6 111.111 419 0112 6 - 614A6 4/11, 61.2.. 111.34 12AT7 3 91 30L1 6 3 .13012 15 1.1.92 5/9 12Ai 6 4 9 301.15 13 9 A, ..211.:N 0Z4 4 6 68E6 41; 61"2, 5.,9 19 43 1,1.1,1 1A3 4'6 &BM 7/11: 6Q7 86 12.11 4 8. 30L17 15 6 111.96 7.1A5 5 5846 8'8 flQ7f, 5,8 30P4 12 A. 6 112AN*6 1A701 7 3 8BQ6 III' 19 6 11L-`,10 9/0 4 916117 11 12AX7 41130P45111 17 6 Aefil. EN 7 6 DM al 1611734 7 NAY; 9 91 lt1 4 9 fiRQ7A 115171 7/8 7 8 68117 15 9168470T 1135 1211A6 6 30P12 13 9 'At' PEN 1. ,5 9'8 6E1116 12'6 681675I 7- 1211E6 5 9 30P10 12 11)6 19 6 1.1'1)1 6 8 8B87 25 68C76T 6 6 121017 6 - 30P1.1 13 9' AC PEN .7, DW4i90031 4:3 611W7 13 686'; 6 12E1 17 301'1.13 15 6 106 6 - 61326 6 - 68117 .s. 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PL303 28.9 7. 1L1.15 8 - fiCH6 6 3,5D5 12 6 A1,60 12BATOT 68X7(11 P61 -3 10 - VP13t; 7 E182CC 2216 ECL811 9 E11.1 3 10 6 PLAOS 97;10 7 9 seLa 8 '6 66Q7GT 7.6 8/- 361.611T 8 614111.3 2 3 111148 1, i 10,- V P41 78 8.236 PABC80 7 3 PL509 28 9 10/6 EC1.84 12 5 1101 5'6 6CW4 12 - 61:40.1 12 128e7 4 6 ATP! 39W4 7,6 1'C'86 10/3 I'L802 15. 9:- VR73 24 8 EA5 6 20.86 11 -112.10 F42 184 41 61)3 71 12807 4/6 34233 10 AZ) 13;- ECI.80 8 E241 8/6 PG89 10/3 015184 7 9 -F80 8.1 51610:, 6,-18.6 4 8 61)6 3- 'woo ; 3 6 128117 84- 3524111 4 9 A7.31 109 6,6A76 1,65 6.9 VR131/ 6 EA1C80 6, 1 ECLLfin0 4/6 8/8 1'112 10 6E1 12.6 , 128.17 4:6 367.8141 8 A1141I 156 a- VT61 A 7, k:AF.ACF9421 103;30 - E281 4:9 0147137 8/6 PY33 10 21)21 6.6 6F6 12 8 ,x, 4 128E7 4 9 50133 6 3 B36 8/8 VC111 7,3 EF22 121 2200 PG900 7/0 PY80 8 43 8A3 10 - 61,61; 5 5 128(171:T9 8/- EF341 53 1,11 10/6 VU120 12.10 8 F1134 8'81W4'11000/8 PCC84 0,1 P1'81 387 5 - 6E12 38 13X3OT 96 81'76 12 6 11117 1211 VU 1201612 7 -10230 PGG83 01182 5.3 4/6 EF37,1 7 8/8 I1,, 3 3136 '9 61'13 36 784 10 8 11/111 io e 141675 PGG198 5;9 .1.-I EVE1 V U131 72/3 EF30 7.6 ER91. G23" 9'9/9 01183 :1Q4 7'6 6F13 107 -6 7B7 tti 19AQ:, 4.9 T2 66 88 51-76 11,. 5 EBC4 I 9/6 F11,40 89 69 10.- 0233 12;8 01474789 91 PY88 3QSGT 8'- I3F19 8 6 I"-, 19111 40 115A2 I::'. 106 WWI 262 2 3, EBC81 EF41 10,- 0231 10. - 266180 10/8 011301 126 95 51 61,23 19 3 7C6 1.7: 201)1 13 84A3 8 8'107 F.RCOO t- E0142 8,6 0237 14.8 PCPS° 6/8 PY800 7,6 :. 7 861 6E24 136 1Ft. 1:: 140.1)-1 205 90A.. 67 6 I> ti '32 11'720 PCF82 14 12 6/8 ISF54 HARCAO 8 6 6;3 011801 6 9 12,8 i:10:1' 10,6 601211 13 - ' 20E2 14 DOA% 67 8 14.51:91 4 3 EBC91 14 . ..7.1 11 13 ,201.1 10. E0173 14'1'94 3,- P230 t. 73 141 5:8 6F.28 8 6 81.41110 96 1., 20 34 1)6E96 6 8 KBP8111 901,1 '1 12 4 9 8.61 59 46 196 1', 196 12.9 QQY03;10 t., 71 arm 38 8 19(1340 10 4)110141683- E0180 . 201.1 17 e 90('1 10 Pc F20013 3 24 YIN 9 166 9 6,131.4211D9 ti 38;*1 51 111180T 3 20013 10 6 1:8119 6/8 8.4113 18 00471 16 - ,D1.14 s' 15 6 00014 10 9 6 X116 7 9 E141.21 "21 V 5 8 14\30927 4 PG.F801 7 EF86 3.9 "11' /.3 9 - (1.1110 18'6 30112 14 6 Dp33 Int I 11 7 6. N 1.01 30 6 /2/8 8.018(1L 1,11412 10 t3 PC1802 9, - Q573 12 6 25 -1101'3 20 Z40 7- 646 3 2 9 -1.33 1304.'2 5 9 DES) £5 5;9;1E3 .101.2 10 64741 4 9 1002 10 251.601 5 6 301 20 - 0106 6 6 E'11,4 101- EP80 5 - HVII2A10 6 PGF80514,- Q8130,15 4/9 EF9) 3'81 IW3 5 6 rermollie 9/-11F1'12 9 6 10 - EC70 12 6 1,, 8 6 4.17GT 6 6 101)" 8 2311 6 352 18 6 00197 iSII 7-6 96 2 6 144'4,360 5 6 PC F808141 QVD4 7 12 6 12/6 E0192 '.3.77 3 8970 2 - 101)2 14 7 251543 8 8 30:1 8 1:1'66 15 - 131163 18 12/- E0197 10 I W4 600 6 PCH20012/3 1110 15,It; 15 -12321, 5 4'- 8K7GT 4'8 10F1 15 -:257.4G 6 - 305 16 6 D1E176 Et P2 L6 EF09 10 6 ((T2 PCL82 71 1111 50 19 8 17 5;-12712 5 ,.11.15 5 6K8(1 4 111018 9 i 25Z5 g - 306 13 _ 011774 KTS 34 6 PCL83 10;- 1110 34;11 1018,20 10 -1 S.K6 '- 61.1 19 4 10E1,, 7 - 25260 8 6 807 11 9 DH81 10 9 ECC3) 15/6 14011133 6 IL6 30431 2 3 61.464/iT 7 9 101,01i 10 6 8 966 2 - 011101 25 All goods are new and mil.Jeet to th, makers guarantee. 11 do not -handle manufacturers' - E5181 E3184 ELT90.1 I - 1 - 1 - :: I F.0 l' nu - - 1 1 - 1 I - -103 1110 ,AQ5 6Alla UAW 6AU6 °AVG 3 3 6L70T 12 6 10213 13 - 3001 13 11891 10 8 1)1110717 11eonds. nor rejects, which are often described as "new 110(1ttsted" 1..ut have al inoltcd and uo10144 20 5 6 61.16, 6 304717 16 - /1763 10 11K32 Leellable Ilk. Busdness bouts Mon. -Fri. 9-5.30 p.m. Eats. 9.1 p.m. 27 6 12166 12 6 361719 14 - 6060 20 - 61.19 5 6 (39 40 10 iTerms bush)... Clash Ith tinier only. rust/paid/Ina ed. p(1. Item. Orders ot,er f5 4 - 61,10120 9 6 12_801 7 6 :9.11 16 17193 10 0 1/1:1)1 5 6, packing tree. Same day despatch by first clam mall. An) pares.] Insured splint (burr.; e in 014 5 6N71fT 8 6 12AD6 7 6 301.1 13 9 7476 14 11K 92 9 ;for only 83. extra. Complete catalogue .4 diner.. transistors fad ecru. P.m. 5 6 ,l't 12 12AE6 12 6 301.L12 18 t1534 20 111E98 price 10d. post free. No enquiries answered unless S.A.E. enclosed f,.r reply. - T.V.T.V. EX -RENTAL TELEVISIONS TWO-YEAR GUARANTEE TUBES "VIDEOCHROME"TUBES FOR BRILLIANCE & DEFINITION I -SEE ILLUSTRATED LIST OF COMPARE TELEVISIONS El I PRICES OP. CHEQUE WITH ORDER TRADE SUPPLIED ALL TUBES PRECISION REBUILT AT OUR OWN FACTORY BY SKILLED CRAFTSMEN WHO HAVE BEEN REBUILDING TUBES FOR OVER 19 10 YEARS EACH TUBE BENCH AND SET TESTED TO A VERY HIGH STANDARD BEFORE DISPATCH PRICES FOR TWIN PANEL TUBES AVAILABLE ON REQUEST 2 YEARS GUARANTEE FREE DELIVERY ANYWHERE IN THE U.K. VIDEOCHROME TUBES LTD. 25 BELLEVUE AVENUE, RAMSGATE, KENT. Tel. THAN ET 52914 10 0 SLIMLINE 405 625 39 gns. 17"--19"-21"-23" 17"-L4-15-0 19"-£5-5-0 21"-£6-10-0 23"-E7- I 0-0 CASH .7--5,_,.;_, 17 SLIMLINE 405 only OUR t .y, II WIDE RANGE OF MODELS. SIZES AND PRICES DEMONSTRATIONS DAILY Carriage and Insurance 301 - TWO -YEAR GUARANTEED TUBES 100 REGUNNED 5,irn Line Tubes 110. 17" and 19" 109 6, 21" and 23" 129,6. Normal Tubes 70" and 90" 17" 99 6, 21" 119'6. 14" and other sizes 79'6. SPEAKERS 101, 2-}" 80, 3-i" 2512. 4" 10(3, 3" 5" 81-2, 7" . 4" 312.8" x 3" 312. BRAND NEW. P. & p. 2/ -Transistors: Mullard matched output >: be RECORD PLAYER CABINET 49 6. 7,'6, OC810-2 0081%. P. & P. AF 117, AF I 14, 0C44, OC4S. 007 I ; 2/6 each Ferrite Rods 3/6: 6" and 8" complete with LW/MW Coils. P. & P. FREE. FREE. Transistor Cases 4 for LI. SIZE. 9+ 446+03/015019/6. STRIP LIGHT TUBES 3/9 each. I I" (284 mm.) 230/240 volts, 30 watts. Ideal for cocktail cabinets. illuminating oictures, diffused lighting, etc.. 6 for El. P. & P. free. ,,-,,. owe _, 4 Cloth covered. Site 161" 7i". Takes any modern changer. P. & P. 7!6. I 4-/ - auto - DUKE & CO. (LONDON) LTD. 621/3 Romford Road, London. E12 Tel. 01-478 600Ij2j3 385 REBUILT TUBES! DON'T BE CAUGHT OUT! YOU'RE Colour Television is already here, but 1970 is a big year for colour television with a number of single standard colour sets coming on to SAFE WHEN YOU BUY FROM the market, engineers with a knowledge of colour television will obviously be in great RE -VIEW! demand. HERE IS WHAT YOU PAY: .. .. .. I2in. I4in. ISin. £4.15.0 £5. 0.0 £5. 5.0 £5. 5.0 I9in. 2I in. 23in. £7. 5.0 £8.10.0 £7.10.0 I9in. Twin Panel 23in. Twin Panel £10. 0.0 I7in. SO DON'T DELAY £5.15.0 We have developed a colour television course geared for the service engineer which will enable him to tackle any problem in colour television Cash or cheque with order, or cash on delivery Discount for Trade * Each tube is rebuilt with a completely new gun The course consists of 10 lessons on colour mix- colour system, colour receivers, decoders, IF circuits, time -bases, convergence, ing, Pal assembly and the correct voltage heater. * Each tube comes to you with a guarantee card covering it for two years against all but breakage. waveforms, set-up procedures, test equipment, fault finding, typical circuits. Fee for complete * Each tube is delivered free anywhere in the U.K. and insured on the journey. course 10 gns. Write for details without obligation to: * Each tube is rebuilt with experience and knowhow. We were amongst the very first to pioneer the technique of rebuilding television tubes. DAYLIN ELECTRONICS RE -VIEW ELECTRONIC TUBES (Dept. A) 32 Parkstone Drive 237 London Road, West Croydon, Surrey. SOUTHEND Tel. 01-689:7735 Learn at home... First Class Radio and TV Cores /10e 9et cW(G) After brief, intensely interesting study undertaken at home in your spare time YOU can secure a recognised qualifi- reflectors. Loft Mounting Arrays, 7 clement. 401, 11 element. 4716; 14 element. 551.; 18 element. 62/6. Wall Mounting 7 clement. 601-: l i clement. 67/6; 14 element. 75/-: 18 element. 82/6. Mast Mounting with 2in. clamp. 7 clement. 42.6: 11 clement. 551.; 14 element. 62!-: 18 element. with Cranked Arm. 701-. Chimney Mounting Arrays. Cornnlete 7 element, 72i6; II element. 801-; 14 element. 87/6; 18 element. 95/.. Complete assembly instructions with every unit. Low Loss Cable. 116 yd. U.H.F. Pre -amps from 75/-. State clearly channel number required on all The DIMMASWITCH is an electronic dimmer BBC - ITV AERIALS incandescent lighting at mains voltages from 200/250 V at 50 Hz. The DIMMASWITCH has the same dimensions (3i" 3-i") as a standard switch and is intended as an alternative fitting, finished in ivory, with a contrasting control knob. Two models of the DIMMASWITCH are now available, each incorporating an on/off switch, orders. BBC (Band 1). Loft 251, External S 0, 301.. £2.15.0. ITV (Band 3), 3 element "1-1-. FREE GUIDE The New Free Guide contains 120 pages ment, Radio and TV. Let us show you how. loft array. 30/.. 5 ele401, 7 element. Wall mounting. 3 element. 501.. 5 element. 55/, Combined BBC/ITV. Loft I -1- 3. 40/-; 1 + 5. 50/; 1 + 7. 60/-; Wall mounting 601.; 1 + 3. 1 + 5. Chimney 70/-: 1 + 3. 70/-: 1 + 5. 80;-. VHF transistor pre -amps. 501-. of information of the greatest importance to both the amateur and the man employed in the radio industry. Chambers College provides first rate Postal courses for Radio Amateurs' Exam., R.T.E.B. Servicing Cert., C. & C. Telecoms., A.M.I.E.R.E. Guide also gives details of range of certificate AERIALS 1+3+9. tronics and other branches of engineer- F.M. (Band 2). Loft S D. 1716. "H". ing, together with particulars of our remarkable terms of Satisfaction or retund of fee Write now for your copy of this valuable publication. It may well prove to be the turning point in your career. Founded 1885 Over 150,000 successes CHAMBERS COLLEGE ilncorp. Na'ional Inst. of Engineering) (Dept. 844V) 148 Holborn, London, E.C.S. DIMMASWITCH NEW RANGE U.H.F. - TV - AERIALS All U.H.F. aerials now fitted with tilting bracket and 4 element grid cation or extend your knowledge of courses in Radio TV Servicing. Elec- ESSEX 75/, COMBINED BBC 1-ITV-BBC2 70/-. 1 + 5 + 9. 80/-. 1 + 5+14. 90/-. 1+ 7 + 14, 1001.. Loft mounting only. 3 element. 57/6. External units Co -ax. available. cable, 8d. yd. Co -ax. plugs. 1/6. Outlet boxes, 51-. Diplexer Crossover Boxes. 1716. 351-, C.W.O. or C.O.D. P. & P. 6d. stamps 616. for illustrated lists. Send CALLERS WELCOME OPEN ALL DAY SATURDAY K.V.A. ELECTRONICS (Dept.P.T.) Monarch Parade London Road. Mitcham. Surrey 40-41 01-648 4884 A capable of controlling from 40-600 watts of and at 101- less in each case a kit is produced to enable a competent constructor to self build the DIMMASWITCH. Both models are available with either clockwise or anti- clockwise dimming and internal fusing may be provided at an additional cost of 3/- in each case, or 2/6d. for the kit. MODEL DS500/2-utilises an advanced design of triggering circuit to enable smooth fade up from zero -E3 12s. 6d. Similar to competitive products at £5 5s. MODEL DS500/2/P-a DIMMASWITCH incorporating a newly patented on/off control enabling the lamp to be switched on and off at any brilliance setting by push/pull action of the knob --E3 I8s. All models give control from zero to maximum and use RCA and Mullard components and semiconductors, and comply generally with the recommendations of B.S. 800. Ideal for T.V. viewing and for children's night lights, etc. Please send C.W.O. to: - DEXTER & COMPANY ULVER HOUSE, 19 KING STREET, CHESTER, CHI 2A H. Tel. Chester 25883 As supplied to H.M. Government Departments, Hospitals, etc. 386 NEW LINE OUTPUT TRANSFORMERS ALBA 655, 656, 717, 721 75/-. BUSH TV53 to TUG69 40/DECCA DM1, DM3C, DM4C (70') 78/-. DR1, DR2, DR121, DR122, DR123 901DYNATRON TV30, TV35 55/6, TV36 70/-. EKCO T231, T284, TC267, T283, T293, T311, T326, T327, T330 55/6. TM8272 68/6. T344, T344F, T345, TP347, T348, T348F, TC347, TC349, TC356, T368, T370, TC369, T371, T372, TP373, TC374, T377A, T393, T394, 433, 434, 435, 436, 437 all at 70/-, 503, 504, 505, 506 95/-. FERGUSON 306T, 308T 55/6 each. 406T, 408T, 416, 436, 438, 506, 508, 516, 518, 536, 546, 604 606, 608, 616, 619, 636, 646, 648, 725, 726, 727, 3600, 3601, 3602, 3604, 3611, 3612, 3614, 3617, 3618, 3619, 3620, 3621, 3622, 3623, 3624, 3625, 3626, 3627, 3629 80/-. FERRANTI T1001, T1002, T1002/1, 11004, T1005 55/-. T1023, T1024, T1027, T1027F, TP1026, 11071, T1072, T1121, TC1122, TC1124, T1125, TC' 126 70/-. 1154, 1155 95/-. G.E.C. BT302, BT304 62/6. BT454DST-456DST, 2010, 2013, 2014, 2012, 2000DS, 2001 DS, 2002DS 85/-. H.M.V. 1865, 1869 55/6. 1870, 1872, 1874, 1876, 1890, 1892, 1894, 1896 80/-. KB OV30, N F70, NV40, PV40, QV10, PVP20 90/-. Featherlight 90/-. PILOT PT450, 452, 455, 650, PT651, P60A, P61 70/-. PHILCO 1019, 1020, 2021 82/6. 1029, 1030, 1035, 1036, 1040, 1050, 1060 82/6. PYE V200, V400, 200LB, 210, 220, 300F, 300S, 310, 210S, 410 70/-. PYE 11U-P/No. AL21003 70/REGENTONE TV403 90!-. R.G.D. RV203 90/-. ULTRA 1770, 2170, 1772, 1782, 2172, 1771, 2171, 1775, 2175, 1774, 2174, 1773, 2137, 1980c, 1984c, 100c, 200c, 2380, 2384, 1984, 1985, 1986, 1980, 1980a, 1780, 2180, 2181, 2183, 2182, 1871, 1783 80/-. LINE OUTPUT TRANSFORMER INSERTS BUSH TV92-TV93, TV94-TV95-TV96-TV97, TV98, TV99, TV100, TV101, TV103, TV104, TV105, TV106, TV1C8, TV109, TV110, TV113, TV115, TV115R, TV' 15c, 123, 125, 128, TV75, TV85, 55/-. Complete with heater windings. DECCA D R95, DR100, DR101, DR202, DR303, DR404, DR505, DR606, 55/ EMERSON E700, E701, E704, E707, E708, E709, E710, E711, Portarama 32/6. FERGUSON 204T, 205T, 206T, 214T, 235T, 236T, 2441, 245T, 246T 30/-. FERRANTI 1472, 14TC, 14T3F, 1474, 14T4F, 1475, 1476, 17K3, 17K3F, 1773, 17T3F, 17K4F, 17K6, 17SK6, 1774, 17T4F, 1775, 1776, 21 K6, 21 K6V 32/6. INVICTA T118, T119,1120 40/-. KB PV40, MV100, OF100, PV100, NV40, NF70, OV30, QV10, 0V30 32/6 pair. PETO SCOTT 1416, 1418, 1419, 1422, 1423, 1716, 1719, 1720, 1922, 1723, 1724, 1725 29/6. PYE V4, VT7, CTM4, TCM7 40/-. REGENTONE 10-4, 10-6, 1021, 17-18, 10-12 30/-. T176, TT7, 191, 192 32/6. R.G.D. Deep 17, The 17, 590, 600, 606, 611, 710, 723 32/6. Guarantee. Post and Package 4/6. C.O.D. 6/-. PRESET CONTROLS Wirewound, 3 W rating T.V. CANNED ELECTROLYTICS . . Ohms 10, 25, 50, 100, 250, 500. K Ohms I, 2, 3, 5, 10, 25, 30. 5/- Each. P.P. 6d. 64-100. 450v. 22,6 each. P.P. 2/6. 100-200. 275v. 18 - each. P.P. 2/6. 100-200. 350v. 22, 6 each. P.P. 2/6. Carbon, + W rating .. K Ohms 50, 100, 250, 500. M Ohms 1, 2. P.P. 6d. 4/2 Each. OPEN (SKELETON) PRESETS Open type controls with mounting lugs to suit printed circuit boards. .. 2/6 Each. P.P. 6d. Vertical Mounting .. K Ohms 5, 10, 25, 100, 150, 250, 500, 680. M Ohms. I, 2, 2.5. Horizontal Mounting .. K Ohms 100, 250, 500, 680. M Ohms I, 2.2, 3.3. Miniature Horizontal Mounting .. 2:6 Each. P.P. 6d. 100-200-60. 22:6 each. P.P. 2/6. 300v. 100-300-100-16. 27/- each. P.P. 2/6. 275v. 100-400-16. 26,- each. P.P. 2/6. 275v. 150-100-100-100-150. 39,9 each. P.P. 2(6. 320v. 15 -each. P.P. 2/6. 200. 350v. 200-200-100. 31/6 each. P.P. 2/6. 350v. 300-300. 300v. 37/9 each. P.P. 2/6. 100-400. 275v. 19/- each. P.P. 2/6. 100 100 100+150. 320v. 39/6 P.P. 2/6. 150 - POWER RESISTOR SECTIONS These wirewound sections enable you to build up any Mains Dropper to your requirements. A central 2B.A. hole is provided for mounting. Ohms 7, 9, 10, 12, 14, 17-S, 20. All at 7A .. Ohms 22, 25, 28, 30, 33, 36. All at 7A. Ohms 40, 47, 52, 56, 60, 63, 66, 75, 87, 100 at 3A. Ohms 120, 140, 160, 180, 200, 250, 270 at 3A. .. .. Ohms 300, 350, 400. 470, 560, all at I2A. .. .. Ohms 726 at ISA. K ohm at -IA. .. K Ohms, .. .. .. .. K. Ohms, 2 (07A.) . .. 2,6 Each. P.P. 6d. Ohms 100, 220, 470. K Ohms I, 2.2, 4.7, 10, 22, 47, 100, 220, 470. M Ohms I. I . "SLIDER" PRESET CONTROLS Wirewound, 3 W rating .. 3/- Each. P.P. 6d. 2:6 Each. P.P. 6d. K Ohms 10, 100, 250, 500. M Ohms I, 2.2. IRON DUST CORES .. 8/- Dozen. P.P. 6d. Dimensions: L.5" dia. 6mm (normal thread), hexagon centre hole. Trimming Tool for adjusting 6mm. cores as above I/- Each. P.P. 4d. 10 -oz. tin Switch Cleaner 9/-. Aerosol Switch Cleaner 17/6. . REPLACEMENT DROPPERS Ferguson, H.M.V., Marconi, Ultra 800 & 850 Series (Convertible) 37 - 31 + 97 + 26 + 168. 12/6 P.P. I Id. As above 850 Series (Dual Standard) 14 + 26 + 97 + 173 12,6 Each. P.P. If-. E.H.T. CAPACITORS 3/I0d. each 4/9d. each PARTS FOR THE CONSTRUCTOR 625 RECEIVER 35/- P.P. 4:6. 90/, P.P. 416. .. .. T.3 Line Output TX T.4 Audio Output .. Electrolytic 100+100+300+16 DI to D4 .. D5, D8, DIO, DI I, b13 T.2 .. . .. 3/10d. each 4/9d. each 3/10d. each 4/9d. each 3/10d. each 4/9d. each Postage each section 6d. Ohms 10, 25, 100, 250, 500. K Ohms I, S. Carbon, 4 W rating .. Single O.B.A. Stud Mounting. 001 of 20kV 10/6 each. P.P. 2;'6. High Voltage Pulse Ceramics. 10, 15, 22, 33, 68, 82, 100, 120, 140, 155, 180, 220, 250pf 1/6 P.P. 6d. L.3 Scan Coils 80/-. P.P. 416. 21/, P.P. 2'6. 27/- P.P. 1,-. 6/- each. P.P. 6d. 2/6 each. P.P. 6d. 2/7 each. P.P. 6d. 1/6 each. P.P. 6d. 9/- P.P. 6d. 6/6d. P.P. 6d. 5/6 for 3. P.P. 1/3/6 each. P.P. 6d. 5/9 each. P.P. 6d. 6/3 each. P.P. 6d. 1/6 each. P.P. 6d. 1/7 each. P.P. 6d. 34/10 inc. 6 10 tax. P.P. 3'6. D9, D12 C35, Pulse Ceramic BSX 21 AC 127 27 Way Tag Strip .. VAI015 Thermistor Fuse Carriers 20mm. S.W.I switch Ceramic valveholders B9A Ceramic valveholders Octal 3 Ohm Elliptical speaker .. Capacitors prices on request. Complete set of 9 Potentiometers 43/8. P.P. 4,'6. CALLERS WELCOME. All new components inserts are guaranteed for three months from the date of invoice subject to the breakdown being due to faulty manufacture or materials. S.A.E. all enquiries. Dept. "R" D. & B. TELEVISION (Wimbledon) LTD. 0 80 MERTON HIGH STREET, S.W.19 01-540 3513 01-540 3955 11. 387 PRACTICAL. VOL 20 No 9 ISSUE 231 TELE11151 JUNE 1910 THEN AND NOW! ALTHOUGH the very first issue of Practical Television appeared in September 1934 it subsequently suffered the indignity of becoming a supplementary section in Practical Wireless. THIS MONTH stimulating sufficient interest however Practical TV in the Dark After the war when it became clear that the re-establishment of the television service was Television emerged from its temporary hiatus and regular monthly publication as a separate magazine began again. Teletopics 388 by I. R. Sinclair R-Y Phase Alternation by G. R. Wilding 395 by Char/es Rafarel 398 That was twenty years ago. In the first issue of the reborn P.T. there were articles on television theory and servicing, aerials, projection systems, broadcasting and constructing a home-brew TV receiver. Familiar ingredients, but with that indefinable tang of the printed word two decades old. The major difference between then and now is the enormous strides which have been taken. Then there were only two BBC stations on the air, 390 DX -TV Servicing Television Receivers GEC-Sobell 2010-1010 series - by L. Lawry -Johns 399 but planning for a nationwide chain was well Power Supply Circuits by S. George 402 under way. Yet there were signs of the troubles to come in the announcement by the BBC that it was not proposed to "change the present system for at least five years." Single -Standard Little did the contemporary writer realise how near the mark he was when he suggested that although the standards of the television service 625 -Line Receiver for the Constructor-Part 4 by Keith Cummins Underneath the Dipole 406 by lconos 412 Waveforms in Colour ReceiversPart II by Gordon J. King 414 Workshop Hints by Vivian Capel 417 Strobe -Trigger Timebase UnitPart 3 by Martin L. Michaelis, M.A. 418 Letters to the Editor 423 recording is a world apart from those early postwar days. One thing however has not changed. Your Problems Solved 425 The first editorial page carried the statement that "Our policy will be technical without being high -brow." This is still our basic aim, though adjusted upwards to cater for the more sophisti- Test Case 91 426 were as good as any home cinematograph" nevertheless "finality has not been reached." Two ideals to be aimed for were given as colour and stereoscopy, developments which "may not reach fruition for many years." Well, we now have colour but the interest in stereoscopy seems to have quietly faded away. In the intervening twenty years much water has flown under the bridge. Today's world of u.h.f. colour on 625 lines, satellite reception which has become commonplace, I.C.s and videotape cated readership of the 1970s. THE NEXT ISSUE DATED JULY WILL BE PUBLISHED JUNE 19 W. N. STEVENS, Editor © IPC Magazines Limited 1970. Copyright in all drawings, photographs and articles published in "Practical Television" is fully protected and reproduction or imitation in whole or in part is expressly forbidden. All reasonable precautions are taken 1-v "Practical Television" to ensure that the advice and data given to readers are reliable. We cannot however guarantee it and we cannot accept legal responsibility for it. Prices are those current as we go to press. All correspondence intended for the Editor should be addressed to Fleetway House, Farringdon Street, London, E.C.4. Address correspondence regarding advertisements to Advertisement Manager, Fleetway House, Farringdon Street, London, E.C.4. Address enquiries about back numbers to Back Numbers Dept., Carlton House, Great Queen Street, London, W.C.2. 388 L L conductive glass across which the voltage is applied. CONTINENTAL COLOUR SETS IN UK With the present shortage of home -produced colour sets comes news of further Continental entries into the UK market. B & 0 sets have of course been available for some time and now German 22in. and 25in. colour receivers from the Telefunken Pal colour range are available through AEG (Great Lonsdale Chambers, 27, Chancery Lane, London WC2. Recommended price of the 22in. model is £355 12s. and of .the 25in. model £381 12s. 7d. The sets have the same basic specification which includes electronic tuning, socket for remote control and external loudspeaker socket. On top of this Granada have placed a large order Britain) Ltd., with a Finnish electronics company for sets for Granada TV Rental. Deliveries of the "Finlandia" 22in. model have already begun. The sets have been adapted by the manufacturers Salora to meet the UK standards and include electronic tuning. Philips expect European colour set sales to grow from 850,000 last year (154,000 in the UK) to 1,800,000 this year and 4,400,000 by 1973. With the forecast that the UK colour set market will reach The voltage and power requirements are low and the displays equally legible in poor light and brilliant sunshine. A new liquid crystal material which changes colour from green to blue when a voltage is applied has been developed at Marconi and present work is expected to yield materials for other colours. The immediate practical uses of the display panels are for control panel readouts, seethrough map displays etc. but it is thought they might one day be used in TV screens thin enough to hang on a wall. The Mullard work is on a transparent solid crystal material _known as KTM whose optical characteristics change on the application of a voltage. This electro-optic effect can be used for deflecting or modulating a light beam, e.g. from a laser, and the low voltages needed with KTN are such that the power and voltage requirements can be derived from portable transistorised sources. KTN optical modulators can be used for laser beam communications systems and are of potential use for flat -screen TV displays. 1,300,000 in 1975, Philips are planning an extension 1 -BEAM EXTEND AERIAL RANGE negotiating Two new aerials-the Metrobeam and Logbeamand a modified version of an existing model have to their Dunfermline component factory and are with Washington Corporation, Co. Durham, for a site for further extensions with a view to this becoming the company's main television component production centre. The idea is to move component manufacture from Croydon to enable television set assembly there to be expanded. been introduced by J -Beam Aerials Ltd., Rothersthorpe Crescent, Northampton. The Metrobeam is a six -element u.h.f. aerial which has RANK -BUSH -MURPHY EXTEND PLANT Rank -Bush -Murphy expect to double their weekly output of colour television sets this year as a result of the recent opening of a £750,000 extension to their Plymouth factory. The output of colour sets is at present running at 1,000 a week and is expected to rise to 1,500 a week over the next few months. Over 5,000 black -and -white sets a week are being produced. The Metrobeam indoor' outdoor u.h.f. aerial. R & D ON DISPLAY SYSTEMS Marconi and Mullard have both revealed the results of research on new display techniques. From Marconi come multi -coloured displays using a layer of liquid crystal only a material called " liquid crystal", a class of liquids with a regular crystal -like structure some of which change their appearance on the application of a voltage. A see-through display panel is made by sandwiching a thousandth of an inch thick between two sheets of been specifically designed for internal or external use. The Logbeam wideband u.h.f. aerial. 389 It is capacitance -coupled and supplied with 18ft. coaxial cable and plug at the list price of £2 5s. The Logbeam is a log7periodic aerial providing almost constant gain over Bands IV and V while maintaining good matching throughout (voltage standing wave ratio less than 1-5:1). The list price is £4 10s. The modification is to the Parabeam which is now a 10 -element model with unchanged list price of £2 8s. 6d. MORE AERIAL EQUIPMENT Two new TV distribution amplifiers-the Castle range-have been introduced by Belling & Lee Ltd., Great Cambridge Road, Enfield, Middx. The Conway is a sophisticated channelised distribution head end amplifier for large blocks of flats and estates and the Stirling an ultra broadband amplifier and power unit for smaller installations Also available is a small, self-contained and economically priced distribution amplifier designed for workshops and showrooms, together with a new range of sockets, diplexers, triplexers and a new non -solder coaxial plug all designed for v.h.f./u.h.f. working. We have often been asked about u.h.f./v.h.f. diplexers. Labgear Ltd. (Cromwell Road, Cambridge CB1 3EL) have available an indoor diplexer type CM6009/DP which can be used for separating u.h.f. and v.h.f. signals fed via a common downlead or for combining u.h.f. and v.h.f. signals on to a common downlead. They also have an indoor two-way splitter type CM6008/TS, an ultra wideband inductive splitter-combiner for combining or splitting any two signal sources covering all TV and f.m. sound transmissions. LATEST STATIONS The BBC -Wales u.h.f. colour service from Wenvoe started on April 4th on channel 44 with horizontal polarisation (group B aerial). BBC -2 from Limavady (Co. Londonderry) started on the same date on channel 62 (horizontal polarisation, group C aerial). The BBC has also ordered a 500ft. mast for a u.h.f. station at Carmel (S. Wales). BBC -2 on channel 63 is expected to commence in Spring 1971. Polarisation will be horizontal and a group C aerial will be needed. The ITA has started transmitting the Harlech Welsh service from Brecon. This v.h.f. relay station transmits on channel 8 with horizontal polarisation and a maximum e.r.p. of 100W. CLE-SOL SPECIALISED CLEANSER A new premium solvent cleaner called CLE-SOL has been introduced by Spectra Chemicals Ltd., Haywards Heath, Sussex. The 18oz. aerosol costs 18/-. Intended for use on electronic equipment, it is inert to acetate and polyester film. and magnetic oxide coatings, cleaning and removing greases, common soils and oils without attacking plastics, resins, coatings, sealants, varnishes, etc. DIY PRINTED CIRCUITS IN MINUTES One-off printed circuits can be produced in only a few minutes by a process developed by Cirkitrite Ltd. (c/o 32 Haven Green, London W5). In this process electrically conductive patterns are pro - The Cirkitrite printed circuit kit. The required pattern is drawn with the pen on specially prepared plastic panels and developed by immersion in a chemical solution. duced by direct application of a special pen to selected areas of the base material. A Cirkitrite kit has been produced (see photo) to demonstrate the principle and includes sufficient chemicals and materials for experimental sample products to be produced. A chemical contained in the Cirkitrite pen is applied directly to a specially prepared material which is then immersed in a metal reducing solution. The chemical provides a catalytical sur- face to the selected areas only so that metal deposition is confined to these areas. Due to the thickness of the copper pattern its current carrying capacity is limited but this can be increased by electroplating or further deposition. Due to oxidation the initial deposit has a limited lifetime. It can however be preserved by further treatment or electroplating. NEW MULTIMETERS Bach -Simpson Ltd. (19 Nortoft Road, Chalfont St. Peter, Bucks) have introduced two new multimeters. The 635 is a 20,00012/V instrument with a basic +1.35 per cent and the following ranges: voltage five each for a.c. and d.c. from accuracy of 3V to 600V f.s.d. with an additional 0-0.3V range on d.c.; current three a.c. ranges from 0.12A to 12A and seven d.c. from 60µA to 12A f.s.d.; resistance four ranges from 0-2MS2. List price is £28. The 635HV is a high -voltage version incorporating an isolated high -voltage multiplier for measurements up to 6kV a.c. or d.c. with a list price of £33. SIMPLE FET VOM Mullard have released details of a simple f.e.t. volt- meter with input resistance on all ranges of at least 10MS2 intended as an educational project. Details can be obtained from Mullard Educational Service, Mullard Ltd., Mullard House, Torrington Place, London WCI. Kits of components are available from sources listed in the booklet. 390 IN Di 1 I 1" I. R. SINCLAIR IF there was one feature which struck the visitor to a TV studio in the early days of television it must have been the intensity of the lighting. The Emitron camera tubes used in those days were extremely insensitive, lenses were of comparatively small aperture and consequently the lighting had to be brilliant enough to compensate for these failings. Much of the work on TV camera development has been devoted to improving the sensitivity of the camera without sacrificing the picture quality in terms of resolution, grey scale or signal-to-noise ratio. There is a limit to what can be done by improving lenses, though the improved sensitivity of modern cameras certainly owes a lot to modern lens techniques; the major part of the improvement in the sensitivity of cameras however comes from the development of camera tubes. The development of camera tubes from the early Emitrons through Super Emitrons and CPS Emitrons to today's image orthicons has satisfied the sensitivity needs of television for all normal purposes. TV however has military and security applications and, at the other extreme of use, nuclear and astronomical research applications. These other uses of TV often demand sensitivity levels capable of televising scenes whose illumination is no more than that provided by starlight, levels of illumination millions of times less than those used in studios for broadcasting. For these special purposes three methods can be us:d to increase the sensitivity of camera tubes. One is to develop the ordinary image orthicon so as to increase its sensitivity without any drastic change in its method of working. Another method, now in production, is to use a variation of the image orthicon design called the image isocon. This is capable of giving better signal-to-noise ratios than the image orthicon at all light levels. The third method is to use a conventional camera tube along with an image amplifier mounted as part of the optical system to increase the illumination of the image applied to the camera tube. THE IMAGE ORTHICON As the first two techniques for the improvement of sensitivity are based on the image orthicon we had better be sure that we understand the operation of the normal image orthicon tube. Referring to Fig. 1, an optical image on the faceplate of the image orthicon causes electrons to be released from each portion of the photocathode, the layer of light- sensitive material deposited on the rear surface of the faceplate. The number of electrons per secondthe quantity which we call current-released from each portion of the photocathode depends on the light strength at that point. The speed at which they leave the photocathode depends on the colour of the light, red light giving slow electrons and violet light fast electrons; this is because the energy of light depends on its frequency. Incidentally it was this discovery which won Einstein the Nobel prize for his work on it in 1905. Infra -red light, being of lower frequency than red, can release electrons only from a very few specially -prepared surfaces. The released electrons are accelerated along straight parallel paths to a glass target which they strike. Because the target is at a fairly high voltage (about 800V) positive to the photocathode, the electrons strike the target at a high velocity-about 20 million metres per second. The effect of this high velocity is that the electrons hitting the target-the primary electrons-knock some of the electrons from the These latter electrons, called secondary electrons, have much lower velocities-in the region of a few hundred thousand metres per second-and are collected by the target mesh which is usually slightly positive to target. Few primary electrons are collected glass. on the target mesh because their speed makes it impossible for them, to change course to strike the metal portions, so most of them pass through. Since in this process more electrons are knocked off the target than land on it, the target becomes positively charged (losing negative is equivalent to gaining positive). The amount of the positive charge varies from place to place, being greatest where the greatest number of electrons have struck. This in turn depends on the light level at the part of the photo- cathode which emitted these electrons. In this way a "charge image" is built up at the target, controlled by the light image at the photocathode. The distance between the target and its target mesh is important, for these two elements act as the plates of a capacitor. If the spacing between them is wide, around 0-1mm. (0.004in.) or more, then few electrons are needed to raise the voltage of the target by one volt because of the low capacitance, and the tube is sensitive to low light levels. If the spacing is close, 0.025mm. (0.001in.) or less, the capacitance is higher, more electrons are needed to charge the target and the sensitivity is lower. The latter case has the advantage however that the charge on the target is large compared to that on a wide spaced target and is also large compared to the clumps of charge in the discharging beam, about which we shall talk later. This gives a superior signal-to-noise ratio and accounts for the fact that close -spaced target tubes are greatly preferred for broadcast use. On the other side of the target an electron beam directed so that it focuses on and is scanned across the target. Because the voltage between the is 391 Target cup Accelerator Faceplate grid 6 Field mesh Decelerator grid 5 Grid 3 Dynode 2 Dynode 4 Dynode 5 Lens Cathode & grid .17 Target Photocathode 1 Grid 2 & Target mesh Grid 4 Dynode 1 Dynode 3 Anode Fig. 1: Structure of the image orthicon camera tube. cathode of the electron gun and the target is deliber- ately made very small the electrons of the beam move fairly slowly as electrons go, at about 500,000 metres per second. Though the target is made of glass or a glass -like material it is very thin (about 0-025mm.) and thus conducts appreciably from front to back so that the rear side is at the same voltage as the front. When the electrons from the scanning beam reach the target three things can happen to them. Some electrons land on the target, neutralising its positive charge and taking no further part in the action. Others, usually the slower electrons in the beam, are reflected back just as light is reflected by a mirror. This process, illustrated in Fig. 2(a), is called specular reflection, so we call these electrons the specular electrons. The remainder of the electrons are scattered in all directions, as is light when it strikes a sheet of white paper; we refer to the scattered electrons (see Fig. 2(b)). The fraction of the total electrons approaching the target that ends up in each of these three categories depends on the charge on the target. When an area of the target has a large charge, because it corresponds with a well -illuminated portion of the photocathode, most of the electrons land and neutralise the charge. Some are scattered and a few are specularly reflected to return in the direction of the gun. When the charge on a portion of the target is low, because of low illumination of the corresponding part of the photocathode, few electrons land, fewer are scattered than in the previous case, and many more are specularly reflected. These differences in the behaviour of electrons at the target are important, because they are the root cause of the limitations of the image orthicon and of the advantages of the image isocon. The electrons which return towards the gun, a # ib a lb Oa (a) the first dynode surface than land on it. The stream of secondary electrons from the first dynode is then accelerated to the second dynode where the process is repeated. At each dynode the beam current is increased about sixfold, thereby amplifying the beam to an extent sufficient to be presented to an external amplifier. The overall amplification in this process is about 500,000 and contributes no noise to the beam. Despite this however the signal-to-noise ratio of the image orthicon is limited by the noise in the beam from the cathode, as this noise is amplified along with the signal. This is made worse by the fact that the maximum return beam is in the regions where the target voltage is lowest, corresponding to poorly illuminated areas of the photocathode. This means that the maximum beam and the maximum noise exist in the black parts of the picture, just where noise is most noticeable. LOW -LIGHT ORTHICONS The standard version of the image orthicon is itself remarkably sensitive compared with earlier camera tubes but quite a lot can be done to improve sensitivity without any change in basic design. For broadcasting use the colour response of the photo- cathode is rather more important than the maxi- mum sensitivity, because each colour must be rendered as a different shade of grey on the monitor. Where this is of less importance photocathodes can be made whose sensitivity is up to three times greater than normal, giving a useful overall gain with no loss of signal-to-noise ratio at the expense of greater processing difficulty and a higher rejection rate. The most favoured high -sensitivity photocathode A a mixture of specular and scattered electrons, land on the first dynode, which is the first of a number of amplifier stages built into the tube itself. Once again the high velocity of the returning electrons, because of the fairly high positive voltage on the dynode, causes more electrons to be knocked from 4, (b) Fig. 2: (a) Specular reflection: for each electron path the angle a equals the angle b. A beam of electrons arriving from a fixed direction is reflected as a beam leaving in a fixed direction. (b) Scattering: the paths of electrons leaving the surface cannot be predicted. A beam of electrons causes scattering in all directions. the trialkali, made by evaporating films of sodium, potassium and caesium alternately with films of antimony on to the glass faceplate. The problems of controlling the process, each stage of which must be carried out with the faceplate at a different temperature, are enormous. So far it has been impossible to form the photocathode outside the tube, test it and transfer it, so that each photocathode is made in place after the rest of The tube processing is complete. Thus a slip up in the photocathode processing means the scrapping of a comis plete tube. 392 Accelerator Demagnifier Field mesh Target cup diode does not cut off abruptly as the voltage on the anode drops and is finally made negative. The reason is that the electrons have different values of energy; some, having low energies, are prevented from reaching the anode when the anode voltage is a fraction of a volt positive. Others, with more energy, continue to reach the anode until the voltage is slightly negative, while the most energetic electrons continue to reach the anode even when the anode voltage is more than one volt negative. Photocathode Target mesh Target Fig. 3: Extended orthicon image section. Another possibility is to use a wide target to target mesh spacing. As explained earlier this means that fewer electrons are required from the photocathode to charge the target to any given voltage, so that less light gives more signal. In broadcast use this has the disadvantage that the beam current from the gun must be enough to discharge the most positive areas of the target, and in this condition the noise of the beam is large compared to the charge on the target. At low light levels the beam current can be reduced, with some reduction in noise, and the wirllr spacing can give a sensitivity advantage up to ten times that of a close -spaced target tube. To increase the sensitivity of the image orthicon much more than this requires some redesign. One method which has been extensively used is to retain the normal structure of gun, target and mesh but to substitute an extended image section of the type shown in Fig. 3. A photocathode of about double the normal area is used; in practice this is done by using the whole diameter of a 44in. tube rather than the smaller portion used on broadcast tubes whose photocathode size is equal to that of the target. With a normal image section the extra area would not contribute to the sensitivity as the electrons from it would not strike the target anyway. However if the image section is modified so that the electrons converge towards the target, all the electrons from a large photocathode can be focused on to a small target. This act of reducing the image size increases the sensitivity and can be combined with a wide target -to -mesh spacing and a trialkali photocathode to give image orthicons of very high sensitivity. An early application of such tubes was in televising the image on X-ray fluoroscope screens. In this way the dosage of X-rays could be cut down to less than could ever be used if the image were to be visible to the eye yet the monitor image could be comfortably viewed. THE IMAGE ISOCON The image isocon has great advantages over the image orthicon not through any increase in In a vacuum-as distinct from the different conditions of the solid state-the differences between electrons of different energy result in the electrons having different velocities, and this means that a number of electrons leaving the cathode at a given time do not arrive together at the anode since some are travelling faster than others. When electrons are leaving the cathode continually, these different velocities mean that the electrons arriving at the anode will arrive in groups as fast electrons catch up with slow electrons which started out earlier. The longer the electrons take to travel from cathode to anode, the more likely it is that they will bunch up in this way, and we must add to this the fact that no cathode emits electrons continually and evenly but only in bursts. We measure this irregular arrival of electrons at the anode as small variations of current which we call noise for the simple reason that it sounds like noise of no definite frequency when amplified and fed to a loudspeaker. Incidentally, diodes are still used in this way as noise generators so that signalto-noise ratios can be measured. The hotter a cathode is, the more noisy the beam of electrons, and even materials which are physically cold can generate noise because of some random interference with the flow of electrons through them. In some cases we talk of these materials as having a noise temperature, meaning that they produce the noise which we would expect a source of that temperature to produce. The lower the temperature, the lower the noise, down to a limit of -273°C (absolute zero) where noise ceases. We cannot reach this temperature in any working electron beam but we can and do run high -gain maser and parametric amplifiers at very low temperatures to reduce their noise level. BEAM NO/SE IN THE ORTHICON The beam of an image orthicon carries a noise signal which is normally inescapable. It arises due due to the temperature of the cathode which must be kept hot so as to emit electrons. Our only hope of reducing the noise is to make the beam forget somehow that it came from a hot cathode! This we cannot do, but we can select a part of the returning beam to form the signal current which appears to have " forgotten" that it came from a hot cathode. sensitivity (its sensitivity is rather less) but because 1a of the great reduction in the beam noise which accompanies the signal at the final anode. In many ways the design is identical to that of the image orthicon but to understand fully how the reduction in noise level is achieved we must be clear about what we mean by beam noise. In any beam of electrons there are differences in the energies of individual electrons. We can illustrate this by measuring the characteristics of a diode valve at low positive and negative values of anode voltage. As Fig, 4 shows, the current through the Va (a) (b) Fig. 4: Noise characteristics of a thermionic diode. 393 Photographs of pictures taken using an image isocon camera tube. Left, picture taken on a cloudless, moonless night. Right, scene illuminated by full moon. Vect.,,n beam- I t ocon Steering plate Target st Electron beams in the image isocon tube. (Illustrations by courtesy of the English Electric Valve Co. Ltd.) The two types of returning electrons are those which are specularly reflected and those which are scattered, as we saw earlier. Now if we compare this with the case of light beams, we see that the beam of light reflected from a mirror carries inform- about where it came from-in everyday language, we see an image in a mirror. A scattered beam of light however does not carry this information, that is we do not see reflected images in a sheet of white paper. In the same way, the specularly reflected electrons from the target make up a beam which has the same noise as the beam from the cathode, but the scattered electrons behave as if they had come from a " cathode" at the temperature of the target. In an image orthicon these two types of electrons ation el of mul are not distinguished. Both land on the first dynode and are multiplied to form the signal current. If however we could separate the scattered electrons from the specularly reflected ones we would have two advantages: first the noise of our return beam would be low because the " noise temperature" of the scattered electrons is the temperature of the target; and secondly as the number of scattered electrons is greatest in the regions of large target charge (white on photocathode) and least in the low -charge (black) areas what noise there is occurs mainly in the white parts of the picture where it is least noticeable. This is the principle en which the image isocon tube is based. There is one slight drawback : the number of scattered electrons which can end up back at the first dynode is small, because 394 Isocons at present in production feature three steering plates so that the steering field can be applied in any direction. The tube is first set up as (a) an image orthicon with the steering plates and separator at the fourth grid voltage. When a good picture is obtained the tube is changed over to isocon operation by adjusting the separator voltage to about 80V and adjusting one or more of the (b) Fig. 5: (a) In the. image orthicon each electron follows a spiral path but the outline of the electrons (i.e. the beam) is straight and focused. (b) In the image isocon the beam as a whole follows a spiral path. The number of spirals has been exaggerated in each case. For further details see illustration on previous page. electrons are scattered in all directions and many end up on other positively charged surfaces. For this reason we might expect the sensitivity in terms of electrons collected per unit of light on the photocathode to be rather less than for an image orthicon, and this is indeed the case. However, the usefulness signal-to-noise ratio than its absolute sensitivity, of a tube at low light levels depends more on its since the image on the monitor must be visible through the noise, and in this respect the image isocon is greatly superior to the image orthicon. The scattered beam is separated from the specular beam in the image isocon by an ingenious method. This involves "labelling" the beam from the cathode in such a way that the specularly reflected beam still carries the "label" but the scattered beam does its not, of the cathode beam. If the beam deflector plates " label" along with the noise from the gun is passed between with a voltage of about 50V between them this, combined with the magnetic field used for focusing the beam, will make the beam move in a spiral path. This type of path must be distinguished from the spiral paths which individual electrons in the image orthicon take. In the image orthicon the spirals are small and the beam as a whole is straight. In the image isocon however the whole beam of electrons spirals its way to the target (see Fig. 5). This spiralling is the "label" we need, because the specularly reflected beam keeps up this spiral motion as it returns towards the first dynode while the scattered electrons do not spiral but return in straight lines. PRACTICAL OPERATION All we need now is a method of separating the spiralling from the straight beams. This is achieved by the deflector plates (usually called steering plates) again. When the spiralling beam of specular elec- trons passes back through the steering plates the radius of its spiral is increased to such an extent that the beam can be intercepted on a plate set at just the right diameter out from the first dynode. The beam of scattered electrons however is given only a slight spiral motion by the steering plates as it returns and the radius of the beam is not large enough to result in interception by the plate (the which catches the specularly reflected electrons. Thus the scattered electrons end up on separator) the first dynode to be multiplied in the usual way. steering electrodes until the picture turns negative. The video amplifier is then switched to reverse polarity and the picture adjusted for minimum noise, uniform shading and best resolution by alterations to the steering plates, separator and grid two voltages along with the usual image orthicon controls. The isocon principle can be and is combined with the features of extended -image section and wide target spacing mentioned earlier to produce acceptable pictures at levels of illumination little greater than that of starlight. The limit of performance is set not by the noise level, as was always the case with earlier low -light level tubes, but by the loss in resolution as the available light level drops. USING IMAGE AMPLIFIERS If the light level on the photocathode of an image orthicon can be increased by any means of the system. One method of doing this is to use electronic amplificathis increases the sensitivity of the image. Image intensifiers were dealt with in an article in PRACTICAL TELEVISION, May tion 1968, but a summary of the facts may be useful. When light strikes a photocathode the number of electrons depends on the light level. When electrons strike a phosphor the light emitted depends on the number of electrons striking the phosphor and also on the potential between the phosphor and the photocathode. This forms the basis of one type of image intensifier in which image diode cells consisting of phosphor and photocathode in each unit are stacked together. A light gain of 50 times in each cell gives a total gain for a stack of three of 125,000 if we disregard losses due to the coupling of cells. In stacks of this type made so far the coupling is improved by making the endplates of each cell from aligned glass fibres, the so-called "fibre -optic" technique. The other important type of image amplifier uses thin aluminium oxide films as electron multipliers either in a straightforward way, by secondary emission, or in the S.E.C. (Secondary Electron Conduction) method of use. The S.E.C. principle was described in an article in PRACTICAL TELEVISION. January 1967. In this type of tube the electron stream from the photocathode is multiplied at each dynode. The stream is kept in focus by a strong magnetic field from a solenoid, which may be an electromagnet or a permanent magnet. The final electron stream at the phosphor is of higher current and also at a higher energy due to the high potential between phosphor and photocathode, so that very high gain can be achieved. The most recent development in very low -light level working is that of coupling an image amplifier of the first type mentioned to an image isocon by means of fibre optics to ensure maximum gain with minimum coupling loss. There will undoubtedly be further progress in this field. 395 G R WILDING Phase Alternation The phase reversal of the R -Y component of the chroma signal on alternate lines in the PAL system serves two purposes: it enables spurious phase changes in the transmission path to be automatically cancelled at the receiver and also enables the R -Y and B -Y components of the chroma signal to be separated prior to detection so that the design of the detector circuits is less critical. In the PAL-D type of receiver these two actions are carried out in the circuits associated with the line -duration delay line in the decoder. THE major difference between PAL and the original US NTSC colour system is the line -by-line reversal of the R-Y information in the PAL system to cancel out the phase errors that can develop anywhere in the transmission path from studio to receiver and which would otherwise cause the display of incorrect colours on the screen. In all PAL -D receivers this continuous reversal of the R-Y signal from line to line is changed back to a- constant phase signal by electronic means involving the use of a delay line. But what precisely does R-Y reversal infer? To answer this question fully it is necessary to consider the input to the delay line. This consists of an amplified version of the received colour sub carrier sidebands minus the blanked out colour bursts. Up to the delay line the chroma subcarrier sidebands vary in both amplitude and phase, the value of the former establishing saturation and angle of the latter indicating hue. After the delay line circuitry-or from pins 2 and 3 in the more recent DL1E type of delay line-what was previously the phase and amplitude modulated subcarrier becomes two separate signals, R-Y and B-Y, amplitude modulated to +1MHz. Subsequent circuitry then detects the instantaneous R-Y and B-Y values by reference to a locally generated 4.43MHz signal which is kept in synchronism with the transmitter by the colour burst signal. After demodulation by means of a special type of detector called a synchronous detector the third colour -difference signal required, G-Y, is obtained by matrixing circuitry which in effect adds an inverted 0.51 of R-Y to 0.19 of B-Y. Concentrating however on the R-Y delay line output, Fig. 1 illustrates the principle of line -by-line Reversal point Reversed phase phase reversal, showing the effect on a sinewave of changing its phase by 180°. This is the change which is imposed on the R-Y signal at the transmitter during each line blanking period. It will be seen that reversing the signal phase amounts to reversing the instantaneous rising or falling polarity values. If the original signal is at peak positive, reversing its phase will place it at peak negative. And if two such signals are added the net output is zero. Equally obvious and vitally important, if a signal constantly subject to phase reversal is applied to a conventional diode detector it will produce the usual positive or negative rectified output according to the manner of diode connection. The fact that the signal is repeatedly reversed does not make any noticeable difference to output. To abstract the encoded information from the R-Y and B-Y signals it is necessary to detect or measure their instantaneous values with reference to a standard, using a special form of detector. The standard is of course the locally generated 4.43MH2 reference signal, the special detectors being synchronous. types to which when the reference signal is applied together with the R-Y or B-Y signal the diodes will be switched on to give an output of amplitude and polarity directly related to the instantaneous R-Y and B-Y values. It will be apparent that whereas the output frorr. the B-Y detector will be a facsimile of the original encoded B-Y information, that from the R-/ detector will be correct on one line but out of phase on the next, so that unless steps are taken to avoid this there will be correct hues on one line and complementary hues on the next. To obtain a constant. phase output, i.e. an R-Y signal as obtained from the camera, one of two courses can be used: (a', line -by-line phase reversal of the R-Y signal it step with the transmitted reversals or (b) line -by-line 0 180 360° 180x` 360 ._/ Original phase Time -0. Fig. 1: Effect of phase reversal on a sinewave. The 180° phase displacement means that the instantaneous positive and negative values become interchanged. Reversal of the R-Y signal phase in the PAL colour system occurs during each line blanking period. phase reversal of the reference signal applied tc the R-Y detector. Both methods will have precisely the same end effect, the production of a constant - phase R-Y signal, and both methods are in use. the latter being more popular on the grounds that it is generally preferable to route the unmodulated 4.43MHz reference signal through the phase -reversing circuitry rather than the 2MHz wide modulated signal. The same techniques can he used in either case, 396 (a )N (b IS No phase [change (a) 11% (b)+kr DC 180° phase change AC input input (b) Fig. 2: (a) Effect of reversing d.c. input polarity. (b) Output phase reversal obtained by switching an a.c. transformer input to opposite ends of a centre -tapped primary. Such transformers, with signal feed or take -off via switched diodes, form the basis of most R -Y phase reversing circuits. so how then is this phase reversal or 180° phase shift continuously achieved on consecutive lines? It is generally done by routing thd signal through, or arranging for it to be tapped from, identical but oppositely wound transformer windings or from a single centre -tapped winding. The basic principle is demonstrated in Fig. 2 where at (a) the application of a d.c. supply, between the transformer primary centre -tap and a free end results in a core magnetic polarity dependent on the winding direction selected by the switch. On then changing over the switch position so that the d.c. supply is connected between the centre -tap and the other free end the core polarity is reversed and a secondary voltage of opposite polarity to that induced by the first application is produced. If an a.c. signal is applied-see (b)-from the centre -tap to a free end the secondary induced voltage will be either in phase with or completely out of phase with the primary a.c. voltage depending on the winding direction. If it is in phase, applying the a.c. signal to the centre -tap and the other free end will result in an out of phase secondary voltage, and vice versa. The steering of the local reference signal or the R-Y signal through a similar type of transformer is achieved by diodes-in place of the switches so far shown-rendered conductive by (usually) a bistable oscillator triggered by line pulses tapped from the line output circuit and with correct switching " sense" maintained by the over-riding ident signal. To turn now to a provided by the DL1E delay line is equal to one line period, i.e. 64,u,secs. The addition of information from two successive lines, i.e. one direct and one delayed feed, doubles the B-Y component since its phase is constant but cancels out the R-Y signal since the addition of (R-Y) and -(R-Y) is zero. This output is obtained at pin 2. On the other hand by rearranging the phase relationships of the signal to give signal subtraction instead of addition the B-Y signal is cancelled and the R-Y signal doubled, but on one line we shall get +2(R-Y) and 'on the next -2(R-Y). This output is obtained at pin 3. Thus terminals 2 and 3 provide pure B-Y and R-Y signals respectively. And in the processes of signal addition and subtraction spurious phase shifts are cancelled. The precise manufacturing tolerance of the delay ensures coincident timing of the direct and delayed line information, while a ma rheostat in line the driver stage emitter 'lead enables exact amplitude matching to be achieved. Turning next to 'the locally generated reference signal, this is taken from the emitter of a buffer stage following the crystal oscillator and fed to transformer _T1 which then induces equal amplitude but opposite phase signals in the two identical secondary windings. The anodes of the two diodes D1 and D2 are connected to opposite ends of these windings while the cathodes are commoned and taken via a parallel RC combination through T2 primary to chassis. The other ends of both secondaries are of the bistable directly linked to the collectors oscillator, the induced reference signals being earthed by the 1.5 kpF capacitors. Rectification of the reference signal produces a positive voltage across the 10p.F capacitor but as the bistable transistors are alternately cut-off and fully conductive during successive line periods to a collector voltage of 14V when cut-off the anodes of the diodes are alternately raised above and below cathode potential to fully bias them on or off. When either bistable transistor is cut-off its collector potential approaches the positive rail voltage since *VVNi .L 022 Pi- +60V 1k B -Y /7077 detector circuit practical example-used in many GEC/Sobel" colour models-illustrated in Technichart No. 1. It will be seen that the delay line driver transistor, which is fed with the amplified chrominance information, is arranged to give two equal -amplitude outputs (a) one developed across its 3901i collector load resistor and fed to the DL1E delay line pins 4 and 5, and (b) one from a tapping point in its emitter circuit and fed to pin 1 (direct feed) of the delay line. (Pin 1 is the one between pins 2 and 3.) The function of the delay line is to separate the R-Y and B-Y constituents of the chromes signal by a process of addition and subtraction. The earlier type DL1 delay line required an external auto - transformer but the necessary windings are incorporated in the type DL1E delay line shown. Delay line action has been covered in these pages before but can be summarised as follows. The signal delay 10kp II la Reference ii oscillator 560 signal ii R -Y switching circuit (ring modulator) fif77 Fig. 3: Reference oscillator feed used in the Beovision 3000 colour receiver. Instead of routing the local 4.43 MHz signal via a 90° phase shifter to the B -Y detector and via the 0°1180° inverter to the R -Y detector, the output from the reference signal amplifier is fed direct to the B -Y detector and via both phase shifters to the R -Y This still preserves the essential disparity between the R -Y and B -Y signals. detector. 90° phase 397 by G. R. Wilding R -Y PHASE REVERSAL TECHNICHART No.1 20V 2 4 33k 1.8kp 5 to B -Y det Subcarrier re ector 390 delay line L Signal feed R -Y det DL1E 390 Delay line driver compensates for losses in delay line B -Y out ut Base of R -Y preamp R -Y out put -J Coincident equal amplitude--' Input from 2nd chroma 120 22p inputs BC108 amp Direct feed to add & subtract circuits in delay line 0.1 47 470 470 5.6k 4.7k 39 Direct/delayed signal amplitude balance /477 47 transformer T1 Ilk II 270 90° phase shifter d burninkginlgine la period 14v tan: 470 470 \A,AQ 270 R1 / 1.5k 180p feed 20V 1k 1001-4o-od\A"rio .05 10-60p line(Reversal occurs) * reversed oscillator -0 1-- 90° adj 560 , Line by 10 01 0A47 Cl 27p 2%0p adjuster T Phase reversing 4.43MHz ref signal from ref osc via buffer stage Direct/delayed signal phase 2kpi 1 100 ills D2 0A47 Switching diodes t 64psec Mil MINI rk 5 p r-kp (one line) 5 680 680 Trigger pulses Bistable oscillator 90° phase changed ref osc feed to B -Y det there is no voltage drop across the 47012 collector load resistor. During this period the diode linked to the cut-off transistor becomes fully conductive so that the induced signal in one T1 secondary winding is fed through to T2 primary. During the next line period this diode is reverse biased, its positive anode voltage falling below its cathode voltage. The induced voltage in the complementary secondary-in opposite phase-winding is then applied to T2 primary. In this way the 4.43MHz reference signal is 180° phase reversed during successive line periods as is required to cancel the alternate line phase reversal of the transmitted R-Y signal, thus providing an R -Y signal of constant phase. As with all diodes arranged to act as series switches for an a.c. signal the forward and reverse bias potentials must be sufficient to ensure that peak positive signal excursions do not exceed the reverse bias or that negative peaks do not reverse positive forward bias. the This possibility is averted the comparatively high (14V) peak -to -peak amplitude of the bistable switching waveform. The two 47012 resistors in series with the T2 secondary feed to the R-Y detector limit the diode current while the 68pF capacitor shunted across detector output plus the series rejector wavetrap filters out the residual 4.43MHz component. by Turning back to T1, it will be seen that a feed is taken from the junction of the preset capacitor Cl and 1.51(0 resistor RI to the B-Y detector. Adjustment of Cl enables a precise 90° phase shift to be obtained. This 90° phase difference between the reference oscillator feeds to the R-Y and B-Y detectors is necessary because of the quadrature modulation system used in the transmission of the chroma signal. Bistable oscillators are generally used for R-Y diode switching since they will rapidly change over from one conductive state to another by pulse injection giving an output with unity mark -space ratio. While the foregoing account of R-Y switching and the manner of establishing the required 90° relationship to the B-Y signal is applicable to most designs, there are several interesting variations. For example in the Beovision 3000 model the output from the reference oscillator is applied to a transistor amplifier which directly supplies the B-Y detector via a transformer in its collector lead (see Fig. 3) but supplies the transformer feeding the R-Y ring modulator-used in place of the two switching diodes in the GEC model previously described-via an RC 90° phase shift combination. Thus the reference signal feed goes unchanged in phase to the B-Y detector but first through a 90° shifter and then through the 0°-180° switch to the R-Y detector. In this manner the essential 90° phase disparity between the R-Y and B-Y detection is maintained. In KB -ITT models R-Y diode switching is accomplished by a high -amplitude ident sinewave instead of the more usual squarewave output from a bistable oscillator. 398 12.00 to 16.00 and 17.30 until start of programme. Tuesday and Friday 09.30 to 11.00, 12.00 to 15.00 and 16.00 until start of programme. Thursday 09.30 to 11.00, 12.00 to 13.30 and 16.00 until start of programme. Saturday 09.30 to 11.00 and 12.00 to 15.00. Colour tests Tuesday and Friday 15.00 to 16.00 and Monday and Wednesday 16.00 to 17.30. Denmark: Monday, Tuesday, Thursday and Friday 09.00 to 10.00 and 13.30 to 17.30. Wednesday 09.00 to 10.00 and 13.00 to 16.00. Saturday 09.00 to 15.00. A MONTHLY FEATURE FOR DX ENTHUSIASTS CHARLES RAFAREL THERE is still no change in the very poor conditions that we have been suffering from for the past two If anything both SpE and Trop reception during March dropped to an even lower level than in February. However it cannot be long now until there is a marked improvement in SpE reception. As I write at the end of March it will soon be April when SpE can really open up. It has done so many times before and even if April is not too good May has always been really open for SpE reception so patience for only just a little longer! I can only suppose that the continuing very cold months. weather has been the cause of the trouble: this winter is the worst that I have known in over ten years of DXing here. As last month, there has been some evidence of F2 activity once again but this too has been on a somewhat reduced scale and limited to the USSR forward -scatter network. It would seem that the recent peak is now passing and the skip shortening as no USA paging stations were heard. Now to the SpE log for the period 1/3/70 to 31/3/70 -and once again I really was trying: 2/3/70 Poland RI, Czechoslovakia R1, W. Ger- many E2 and " new" card E2 see below. 5/3/70 Poland R1, Czechoslovakia Rl. 10/3/70 Czechoslovakia Ri. 15/3/70 Poland RI, Czechoslovakia RI. 18/3/70 Poland RI, Sweden E2. 21/3/70 Czechoslovakia R1, _Sweden E2. 29/3/70 Poland RI. 30/3/70 Czechoslovakia RI. There was F2 reception from the USSR on the 3rd, 6th, 14th and 20th but with weaker signals than last month. The less said about the Traps during this period the better! There was only the odd day or so when things temporarily improved. Thanks to G. J. Deaves of Norwich we are able to publish an official list of Norwegian TV stations as follows (all Band I and horizontally polarised): Ch. E2: Melhus 100kW, Steigen 60kW, Greipstad 60kW, Varanger 30kW. Ch. E3: Gamlemsveten 60kW, Bagn 30kW, Hemnes 60kW, Kautokeino 8kW. Ch. E2: Melhus 100kW, Steigen 60kW, Greipstad His Scandinavian source of information also gives the following times for test card transmissions in Norway, Sweden and Denmark: Norway: Daily except Tuesday 09.30 to 17.00. Sweden: Monday and Wednesday 09.30 to 11.00, I presume that the above times are .in GMT but it is not indicated in the Norwegian publication. The rest of the information I assure you is correct even if it was translated from the Norwegian by " yours truly" based on what knowledge of the language he picked up during his holiday there last year! Roger Bunney has also received an official list of Finnish TV stations as follows: Finland: 1st programme Taivalkoski E2 15kW vertical polarisation, Tervola E3 80 kW horizontal, Kajaani E4 15kW horizontal (TV1 on card). 2nd programme Tampere E2 10kW vertical (TV2 on card). His contact in Finland says two new u.h.f. stations are expected to be on air shortly.. These are located at Sippola and Jyvaskyla, both in Southern Finland, so could be possibles here if conditions are " excellent". The channels are not yet known but he hopes to have more news soon which we will pass on. There is one Finnish u.h.f. station listed at present. Lahti Ch. 40 (horizontal) but it seems to be low power at present -only lkW. I would suggest that this is due for uprating. Finland is a long way for u.h.f. but it could happen here. We have already had our first 1970 mystery received by Roger Bunney and myself in the form of a new test card on E2 on 2.'3/70 at 11.52. Unfortunately it was a short duration signal with no follow-up at 12.00 in the form of captions etc. This test card consisted of two concentric narrow black circles enclosing a single horizontal contrast wedge in the lower half of the circle and a white square inside towards the top left-hand side. With the weak signal there did not appear to be any corner circles on the card. We have no idea as yet as to the origin of this card. From the aerial direction it seems to lie to the North-East and we would be very grateful for any news of other DXers' reception of it. Now for a very preliminary report on high -band Meteor DX, the subject of our recent articles. The results to date by Roger Bunney and myself have been encouraging if not exactly spectacular. We chose Band III Clis. E5/R6 and there has certainly been some evidence of very short duration activity in the form of snatches of programme and sawtooth patterns. No positive station identifications to date but if we can get some bursts of test card we will know the source of the signals. We are continuing our vigil and hope to have more news before long. We feel sure that this new project is well worth further investigation. May I ask you all once again for your co-operation. Your reports on this type of reception would be most welcome as we are anxious to assess as far as possible the potentialities of this new method of propagation and reception. 399 IN SERVICING television receivers L. LAWRY-JOHNS GEC-SOBELL 2010-1010 SERIES which was a reasonable change and all our heads nodded in agreement. A couple of years ago our WE dealt with the Sobell 1000 series in the June -July This later group of models was 1968 issues. heads stopped developed from the earlier chassis and many of our notes will be found applicable to a very large number nodding. turret was The small replaced by a strange contraption of springs, iron dust slugs on wires, sliding contacts which are almost inaccessible, tiny screws of differing lengths which have to ..c.To in their own tiny holes, pieces of sticky of models produced from 1964 up to very recent months. There have been many minor circuit changes and the layout differs considerably but the fault symptoms and their location remain sensibly the same. For example, the dropper sections still go open -circuit although the dropper is no longer at the top but at the left-hand side of the chassis. The horrible disc -type thermistor (TH2) used in tape over resistors which change value and a kinky disc which wanders out of position to make channel changing a chancy affair. Despite this however the range of receivers is among the most reliable and, perhaps more im- earlier models from the dropper to the rectifier, and which was a real drop -out, has been replaced by a more sensible tubular type which doesn't give any portant, the most predictable available in recent trouble. For example the Sobell 1020, GEC 2020, McMichael 3020 and Masteradio 4020 are all fitted with the same chassis. With certain reservations these notes can be extended to the 1010, 1012, 1017, 1018, 1022 years. The system of numbering the various models under the different brand names is logical enough. We cannot wax lyrical about the v.h.f. tuner changes however. The original rather bulky semi incremental type with the two large discs was super- seded by a more conventional biscuit turret type R158 PICTURE 245 226 2R5 245 326,205 CENTERING VOLTAGE SELECTOR (S:51)t144. UHF-- r -s TUNER R32 75.r, UHF 1 SOUND OUTPUT TRANSFORMER AERIAL INPU SOCKET C 3$ 19 .n.. VHF AERIAL INPUT C13 71 L-' 1 FS.1 1.5 AMP MAINS FUSE :T NI SOCKET r - 1- VERTICAL LINEARITY SMOOTHING CHOKE /OVERALL FRAME OUTPUT TRANSFORMER TOPI C189 + C 190 - -L-74 1 I I i I PS I/F PANEL R2- 0 7 PS 01 P10 _ T/11 PAN 9 " T12 --`-- Pll 405 HEIGHT VERTICAL HOLD 625 HORIZONTAL HOLD SET BOOST Fig. 1: Rear chassis view. In earlier models the focus adjustment (R137 -R139) is on the tube base. Z.; I+ _iy- tz, 400 6:177 HT3 PC13 SW2-9 2.2k HT5 1C82 Or R38 PCB 1.8kp I Sp C70 R63 3.3k C851 1131Z R46 8p IC90 Z.5kp 78 33k 75 C 1 C93 32 45L ffozr C79 6p 160 pH R71 66 35r T27 LL Pcsek:= /7777 15k 41.4MHz rIC72 LC101 68 aSkp V3 EF183 -3 7 9-4= 405 TI 0. R72 405 SW2-6 1.3 0.22, C9, 2kp 10 V5A PFL :200 86: 10122 82 SW2-8 0.1 6MHz R60 27k C99 R66 R41 27k 76 1.5 PC22 SW2-5 1.214 R48 27 857 3.3 16D 200p 3.5MHz kp pc'n. kp 95 1.5 2.2M R62 1.2M m4C92 ff C67 I .54 91. Cs. J. R49 82 Jo 22 oe T1.p5 RS9 ISO ;00 OS R70 150 1C69 .1'0.22 PC10 C.. R50 12M Link R40 A 820k R42 1-5M R45 220k 1 12V NM, PC11 Tit low R32 29k R31 E (c.r.t. cathode) and 1038 series, bearing in mind the fact however that the 1012 group feature completely transistorised tuner units and i.f. stages with a heater circuit diode to supply current to the six valves and tube. The 1017 features v.h.f. radio facilities and a ratio detector circuit as opposed to the quadrature EH90 detector used in the other valved models. Common Faults The most common fault which will be encountered is a failure of one section of the dropper. This is usually towards the front end and is most often the The symptom is that although the 1541 section. valve heaters are glowing normally the set is otherwise dead. The faulty section can be located immediately with a neon screwdriver, a healthy glow at all sections indicating that the dropper is not at fault so that one moves on to the thermistor TH2. However it is the dropper which will almost certainly be at fault and one has the alternative of fitting a complete new dropper or bridging the faulty sections with a resistor of the correct or near value and an adequate wattage rating. The writer's own favourite is a Radiospares 140 section which can be fitted in minutes with two small nuts and bolts to ,make a neat and strong job. If a wire -ended resistor is used the wires should be wrapped securely around the tags before soldering and it should be of IOW rating or higher. It is not clever merely to short out the faulty section by means of the flying leads and tags as this raises the h.t. voltage and causes further trouble. If the symptom is valve heaters not glowing check the rear sections of the dropper, remembering that these have a much higher value, and if all these show a strong indication move on to check the PY800 heater. Fig. 2: Circuit diagram of the receiver stages, 101, 2-2k Neon checks are only useful if done properly (however simple this may be). The first check must always be to the receiver chassis to ensure that it is not " live". If all the on -off switch and the mains supply as the neutral is likely to be open -circuit. Distorted Sound Low and distorted sound is most often due to the resistors associated with the EH90 valve changing value. The cause of the trouble is mainly the 18kn 2W resistor R92 on the left side of the panel. This gradually falls in value causing an increase in current which cooks it further until the value falls rapidly causing the 5.6kn resistor R93 to overheat and change also. The smaller 18051 resistor R90 in series with these two seems to escape damage due to its already low value. The distortion usually calls attention to the situation before the current flow becomes excessive so that the danger of a complete h.t. short and burn -out is averted. The values are not too critical and a 22k11 resistor can be used in place of R92 if a large 18k11 resistor is not available. Is is essential that this is rated at 2W or more. The rating of R93 is not so important as less voltage is dropped across this. A 4.7kn 1W type can be used if the correct value is not available. Distortion can also be caused by the limiter (GR3 0A81) clipping but this is not usual even if the 4.7Mn resistor R84 goes high because of the 1.8Mn resistor R87 from the EH90 anode. This resistor serves to " lift " the 0A81 away from its limiting condition on 625 since the voltage at V7 anode rises to 125V on this standard. We do not imply by these remarks that R92 and R93 are always responsible for distorted sound, 401 L1-240 HT3 sep output .)---o- Video output C125 PC11'. PC17 !). 1 PC18 AGC )1 R84 4.75 C107 R77 33k lSkp PL1-6 R87 1.85 ki 058 C104 PL1-9. PgAL at.C13 330p 6.8k43 673 080 C124 12T). 9 -78 47k R95 HT2 I7k r C117 01 LI-7* -, Is PLI-84 CD 376 GR3 1109 R91 0081 47:8 C121 22p L47 16p 33 PL2 2 1 P2 R75 0106 2.25 IS 930 SW2 ci, R85 C112.... 1. 5kp.0 131131 15 Lin contr..st 01214 PL1-5 1 /47 SW2-3 32p P1 500k C132 R96 1C8pT 1141"." T15kp C115 SOOk Log volume 685 C111 0.22 3 180.5 01 1Skp.1. C118 .129 56" 690. C120 04 R941 4 50 31619 PL2 T LISP local PC11.)_ R56 12M R52 2714 V" UHF normal P11-4 GEC-Sobell 2010-1010 series. next month. i/ There are a number of component value changes in later models. HT3 These will be listed (DI STANT ) l't4/1°C.7 -' R40 '''' ''' C13 i__t. 4 4, (44,586 i (PC7 /..._,_, 2 4 0.0(4-4 i i al 3 c 1 41 ...t. 1 r -I, C657 LV4312C0 s /C13.r ''\k,, tki i .--(wa pca 7 , R 95 4 I ft I 4 5 t 072.07 i Ili F t-,..,,, 9. C80 aH-4* ----. Z.76,r 1 2 ..... ..11.. ,..y.2z2. N-------i 91 6'4 ,..,,,fri tric .....4_) j!,\P I ' - ---. 5 .t68 Z 1 C11121, 1 tc-r-'i.ii,i4.1.f ik. 1 3 24, ---4---- RV,R89ic. .,..` tC1 4 3 ,.. * ! --- .t. 241, .-II I.' !c 7 V; "1.- .:*-4--wo ....N, R85 ilei42 ....., CR31 ,,,,..ig '\ I I C1321 C107 'IP* ''.: 1P-i IF 4. R84. -' Pip SW2-5 Y \ \ WI I. , 88 N. Ile . I '%t R 83 " ), . . 1 4 .1-'-'41. C128 ... ...,,' .00r10 ii. F.. 4o / ,e--. DOO \--i L43561 'N XI 6_15,4,,20, 1,-6, 1 to -107,04 R60 L42 .---ett.v---w..-- ieralkr.. . \ /SW2-841 At /7777t'''4 c97,,,, p Td._ 99/To-'*. : I/ i 14,5 ....c 911R 674 '...2'2%***. R63 j.IL.k---+*9W----41 C115.4,...4", ...."**"'"" ... - ...01')CL2s. I ....::LA,14.:-4,,,-). * 1 . 73 C4 1 9 17. 4.'IR 1 1 . ....,,,,....R e...- _r zz A 7 Vd 8 9 \/1f112 , ***. 1.41 81.4, 'c% 1 \ CSTii rw ......1 /1 ,d,.5 \ "-I ----i\ e ... ...9 \ ...C(.74-The.:64 `' ic:31,1-11 i C102 N......o' * a n 414,-w 4 )iik.V.--11-_____v_____. ..1,:w..4. R 69,, ...,. , ) . c I 5W2-7 X`'.."-, *--At-1667,____ is, P81 14. C90 1, I ,`'"" 5W2-6 I"''''.112.7.1:8 71 cr3N"."-., .5.. ID -1 **. \ I" 9,78 7-"'cl;---3 ..--..." 7. ---;15 ..__\._ voi ''',E., .GA.1'17 IR 8 2 Etiepaeit. -7 ,_ k -7;;I ... 141........10.--11--. 401111-8...4 Foi, \ c i 4 if - ---.--. --J C 1 C / + g ..... 3 R90 f I Ir ,.. 'f 1 I 0 i .26 1\ 5V . 7T \ (1 4 ,SW2-3 i...."Z-1 ...4 ti .) : 5-dp C83 ,4,i,-- '""....7.."'"'''''''....# 0111 = ....- / - * ---- (---, C122 4/-11---40 TS r'7.1.44-7.-ivz ..-r-c,ii. i *.-r--. R92 ,...-.'4.-1 if ,,,,ss W 2 -2 -I ' 0,... .--).-- 941.61.1.31, -. 1 / 1 _9 I C116 / 5TC 73 k2 ' 1 L__Cil__ ...4 s, 1 , L.38 I 74 1 ...._...,..._ ................ ....fr.--, 11-1....-.--411 C131 9p/ IC 139 /- R58 ,37;,3>.-r,4-61,892,77, ) Tt. _,....I P R59 7/64, 2., 0 7. 4T.7_14-: 'Pr,. '57 --' 43i i ,') 4, '4 ....._ . ''''''' ."-- 1.414 1 3Zire ! -, 4 2+!i4fi'''1 R44-1.- 1' __N ,\ 1, ' k% 4-7,f ,..... 1,....mc- 12.'....-9.ja.' .. ZF6 7 /R. Ir -----'-7,..1---_ I 44.p..r .... 7-- '''''' R 1 pc,5 .1 7 .1. /-....6-34Q# ,.>, I t'Vpc12 --- R90 R ',45....,..4....4.4iiii,......... .0,5,...,5.. NINO Miiiiailliiiiiiiii 10....... '.."' ..'. ..." "...""' '.".".. ""..... As,. ip C6::; c 13***32,\...N..... ' 41*ii.pz2 ..C11..,07 'C1 '*****3. C93 I c'°4 \ L *NYIHI-t ',4 Fig. 3: Layout of the i.f. board viewed from the print side. Wiring on component side shown in broken lines. Indeed it could be said that the PCL84 valve is just as often responsible for the condition. Quite a few things can happen to this stage but before discussing these let us be quite clear on one or two points. The output valve used on the 1012 series is a PCL86 of a PCL84, triode as a.f. amplifier as there is no EH90 in this while the 1017 uses both sections model. All other models use the pentode section as the sound output and the triode section as clamp diode (grid and anode strapped). a.g.c. The usual troubles in the output stage are low emission, making the sound weak or non-existent, leakage between electrodes which causes low and distorted sound (R92 being quite innocent despite its discolouration), and shorts between electrodes which cause the bias. resistor (R96, 1505 on some models, 120n on others) to burn out. In the event of the latter resistor being found charred replace the PCL84 and check the 25/-GF electrolytic C134 which may have suffered during the fault condition. As far as the replacement of the resistor is concerned the follow-continued on page 424 402 POWER SUPPLY CIRCUITS nnn TELEVISION h.t. supplies are almost universally provided by means of a half -wave rectifier, either directly fed from the mains or from a tapping on an autotransformer. The size and cost of a fully wound transformer usually outweighs their advantages in standard monochrome receivers, but in colour models a transformer is generally employed in order to conveniently obtain the transistor 1.t. supplies and the heater feed for the shadowmask tube. Whether or not a transformer is used for the h.t., shadowmask tube heaters are always transformer fed to eliminate the risk of switch -on surges that inevitably occur in a series heater chain. H.T. circuits can be of two types, (a) single -rail versions, usually employing an iron -cored choke for smoothing to minimise the voltage drop, and (b) arrangements providing multiple h.t. rails each individually fed by low -value wire -wound resistors. This latter arrangement reduces the risk of unwanted coupling between different sections of the receiver and has the advantage of providing differing smoothing and voltage levels to suit the requirements of different stages. For instance the line output stage, which requires maximum voltage, can be fed via a lower -value resistor from the reservoir capacitor than the signal amplifying stages for which the smoothing level is the most important Peak voltage = 240V x 14 = 336V Positive 2I cycles of mains supply Reservoir capacitor voltage Ripple voltage 4 200V °I'vv`rr oT T. S. GEORGE smoothing. filter will be unable to maintain a constant output voltage. A rectifier can only conduct when a positive If the latter is assumed to be 225V, then as the peak 240V mains input equals 336V (240 X 1.4) voltage at its anode exceeds the cathode voltage. diode conduction will only commence at approximately sin 225/336, or closely 41' from the commencement of a positive half -cycle, and will finish slightly earlier than this angle from the termination of each positive half -cycle since the charge will have raised the reservoir capacitor potential during this time. Therefore, assuming that non -conduction occupies 80° of the positive half -cycle, conduction will only be for 100° of each complete cycle and the charge current must thus average three-four times that of the constant current drain with peak values rising to a still higher figure. The reservoir charging current thus consists of a train of comparatively short-term, high -amplitude, pulses: Fig. illustrates these points. rounded 1 Throughout a one -cycle period the total charge drawn from the reservoir capacitor will be Q=It, and assuming a current demand of 300mA this equals 0.3 x 0.02 or 0.006 coulomb. With a typical reservoir capacitor of 200p.F charged to 225V the voltage drop at the end of a one -cycle period equals Q/C. As this basic formula relates to capacitance in farads it is convenient to convert C to micro coulombs. Thus the voltage drop is V=6,000(µC)/ 200µF =30V. This variation or a.c. ripple must of course be reduced to a fraction of this figure by the RC or LC filters. Such filters can be considered as series RC or LC combinations with the capacitance offer- OV ing a low reactance to the ripple or a.c. component t 360° 360° 360° and the resistor or choke offering a much higher .1 impedance so that most of the a.c. content is developed across the inductive or resistive com- t = rectifier conduction periods Fig. 1: Half -wave rectifier output. As diode conduction ponent. voltage ranging from just over 200V to a maximum of about 265V. The reservoir capacitor associated Chokes are much more effective than resistors since the resistance of the latter represents its sole impedance to a.c. or d.c. while the reactance of a choke to a.c. can reach a very high value, with the added advantage that the d.c. voltage drop across the winding resistance is small. While chokes are often used with a single h.t. rail system, resistors are generally used in multi -rail circuits mainly because of cost and size considerations. At this point it is appropriate to see how the smoothing effect of these filters can be calculated. the stipulated voltage with only a slight voltage drop between the succeeding positive half -cycles or the the individual reactances of the two components XL and Xc are electrically opposite, so the formula to occurs for only about 100° of each full input cycle the reservoir charging current is on average three-four times the constant receiver current. factor. When a single h.t. rail is used the smoothing level must of course be adequate for all stage requirements. With either system the total h.t. consumption will be in the region of 275-300mA with the output with the rectifier must be large enough to sustain For LC filters the main point to be made is that 403 Smoothing 1000p choke Diode protection 240V 22 300 To smoothing TSmoothing capacitor filter Surge limiter 200 TReservoir /477 17177 (a) ( b) Smoothing Smoothing 0.220V resi tors resistor P. 205V 800 200V 84 120 150 Smoothing Tcapacitor /47 /T150 T350' /477 (C) (d) 14 Smoothing capacitors 182 HT1 Smoothing section Surge limiter section 220 3001 100 240V HT4 230V 3.3k ,mMeNAA.,411immmimw. HT2 215V 2k HT3 235V Reservoir capacitor /477 32 32 4=1 100 capacitors (e) Fig. 2: (a) Basic diode rectifier circuit. (h) -(d) Three common types of smoothing filter. (e) Bush -Murphy h.t. supply circuit with four -separate h.t. lines and surge limiter in cathode lead of rectifier. HT1 feeds the tuner, LI stages, video and line output stage; HT2 feeds the line oscillator; HT3 feeds the audio output stage and HT4 the field output st age. determine by what percentage they reduce input ripple is as follows: Output % ripple - Xc X 100 Input XL-Xc When the reservoir ripple is known therefore becomes a simple matter to calculate it the ripple voltage across the smoothing capacitor. As on the other hand resistance and reactance are electrically at right angles, the formula for an RC filter is: Output Input % ripple = Xc x 100 V(R2+Xc2) An idea of the effectiveness of simple two -component smoothing filters can be gauged from the figure relating to the h.t. supply system used in many GEC models: the potential of 267V across the reservoir capacitor has a peak -to -peak ripple of 30V while on the receiver side of the choke the ripple is reduced to only 1.7V peak -to -peak. A further RC filter comprising a 3000 resistor and 50µ.F electrolytic capacitor feeding the vision and sound -receiver section then reduces this ripple to . only 0.3V peak -to -peak or 1% of the original figure. Due to the high value of electrolytic reservoir capacitors used and the extremely low forward resistance of modern diodes, surge limiters are absolutely essential to prevent damaging current surges at switch -on when there is zero charge on the capacitor and the initial charging current is solely limited by series resistance in the circuit. Their value is largely dependent on reservoir capacitor value and what other resistance may be present in the circuit. Where the h.t. feed is from an auto or fully -wound transformer the surge limiter value can be greatly reduced or it may even be dispensed with altogether if the d.c. resistance of the transformer winding is of sufficient value. Although surge limiters are usually connected in. the a.c. feed to the diode anode (see Fig. 2 (a)), this is not universal practice. In many Bush -Murphy models the surge limiter is connected between the diode cathode and the reservoir capacitor (see Fig. 2(e)). With both arrangements their action is of course the same. In a valved receiver employing a semiconductor rectifier the reservoir capacitor voltage will rise rapidly to the peak applied voltage until the valves start to pass current. Thus the rectifier must have a working voltage well in excess of this value. Further- more when the diode anode voltage swings to the maximum negative value the reverse potential across the rectifier will be twice the peak voltage, or about 672V, so the rectifier must be able to withstand this considerable voltage plus a safety margin. Hybrid and Colour Receivers Most monochrome receivers now employ transis- tors to some extent, either in an integrated tuner d() d PL504 RS A.0 input from R3 CRT 110 Heater chain 25 mains input circuit ECC82 PY800 47 Fig. 3: Negative transistor supply derived from the rectified heater supply (Pye). The positive temperature coefficient thermistor R4 prevents switch -on surges developing excessive voltage -18V l.t. output Smoothing R1 1000 C2 1000 Reservoir Smoothing C1 R2 350 across R2. /7777 6 S10 GR2 290V NV's. I Surge limiter To solenoid -1 2 2kp 3k "'Smoothing 300 ak Reservoir switch and 2.5A 700 190V 15k auto degauss circuits Decoder plug contacts .W.I111>< -.4, I NM 0- 20V ii 0.1 196V AC to valve heater chain II II II -ID- 15V II 2500 II Reservoir 11 20V AC = 2500 II Smoothing CRT heater 125 100k 68 NM, Surge I5° Reservoir 0A91 14 1. 50 Smoothing 6.-20V 560 Smoothing limiter Fig. 4. Power supply circuits of the KB -ITT Model CK400 colour receiver. or throughout the vision and sound i.f. stages. When the current demand is small, as when only the tuner is transistorised, the l.t. is usually provided from a potential divider between the h.t. line and chassis. When the complete receiver section is solid-state, however, many designs derive the I.t. supply from the rectified heater current by including at the end of the chain a low -value resistor or resistors across which the desired 1.t. is developed. This rectified heater current consists of a train of positive or negative half -cycles depending on the heater rectifier polarity, and must be well smoothed before it can be used as transistor power. As the circuit impedance is low this implies high capacitance shunting electrolytics and a small -value series smoothing resistor as shown in the typical example in Fig. 3, used in many Pye hybrid models. R I, R2 and R4 constitute the heater chain termination with R3 in conjunction with C1 and C2 forming a pi smoothing filter. R4 is a positive temperature coefficient thermistor so that when it is cold at switch -on and the heater current is high its value remains low to reduce the voltage developed across R2 and thus prevent excessive voltage being applied to the transistors during this initial period. When the valve heaters are fully warmed up and the heater current falls to the normal value the resistance of R4 increases to permit the correct voltage to be developed across R2. This action is of course the opposite of that given by a negative -temperature coefficient thermistor in series with a heater chain. Due to the hybrid nature of most colour receivers and the fact that both npn and pnp transistors may be used in one model, both positive and negative 1.t. rails may be required and at differing voltages. Fig. 4 shows a typical colour receiver power supply system giving h.t. at 290V and 190V directly from the mains input, l.t. at +15V and +20V from a bridge type transformer -fed rectifier and -20V derived from a tapping on the transformer primary. The bridge rectifier in conjunction with the large -value and smoothing capacitors gives an extremely high smoothing level, while the secondary winding resistance obviates the need for surge reservoir limiters. A miniature thermistor is included in series with This enables a the 612 .h.t. circuit surge limiter. slightly higher d.c. output to be obtained than if only a resistor is used, since of course the thermistor's value reduces when warm to marginally increase the applied a.c. potential. FAULT SUMMARY Reduced h.t. with increased hum level This may be caused by (a) a reduced -value reservoir capacitor or (b) excessive cu. -rent consumption. 405 The two most common causes of (b) are complete lack of drive to the line output pentode or a valve with an internal electrode short-circuit. To check for the latter-if no overheating resistor indicates the faulty valve-connect an ohmmeter from the h.t. rail to chassis and remove or replace each valve in turn till the short-circuit meter reading drops significantly. Care must be taken when making resistance tests from h.t. to chassis in modern receivers with low resistance rectifiers, for if the ohmmeter current is of appropriate polarity it can flow through the IN NEXT MONTH Practical TELEVISION rectifier and then via the heater chain to chassis to give the erroneous impression that a short-circuit exists -when none is in fact present. Furthermore if a partial short-circuit does exist it will mask the effect of removing valves and disconnecting any circuit feeds to trace it. An essential precaution therefore is always to reverse the meter leads to obtain the minimum reading across h.t. and chassis or to remove any one valve or the c.r.t. base connector to break the heater chain. Normal h.t. with increased hum level In receivers with a single h.t. rail increased hum level is often the first sign of a deteriorating smooth- ing electrolytic but on occasions weak field sync may be the most noticeable symptom. Where there are multiple h.t. feeds the symptoms produced will depend on the type of circuit decoupled by the faulty electrolytic and may be loss of sync, instability or impaired picture quality, etc. In all cases the quickest and most positive way of identifying a faulty electrolytic is to temporarily connect a known good one across it. This test capacitor does not need to be of equivalent value, in fact a truer test is made with one of smaller capacitance since only very rarely does an electrolytic go completely open -circuit. It is important not to stab a large -value uncharged electrolytic across a suspect in a working receiver as the initial heavy spark and surge current could damage the rectifier and possibly a circuit transistor as well. The safest procedure is to connect the electrolytic across the h.t. rail and chassis before switching on so that it can charge up slowly. It can then be subsequently applied to any high voltage point with minimum voltage disturbance. On rare occasions increased hum level can be caused by short-circuited turns in the smoothing choke which reduces the inductance and therefore the reactance of this component to the supply a.c. ripple. This usually results in the choke running COLOUR FAULT-FINDING -MAKING A START Specially prepared for those who have not so far made a start on fault -tracing in colour receivers this article also provides many tips to help those who have already plunged into the complexities of colour sets, with a view to saving the hours that can otherwise be wasted following wrong trails. A great deal can be learnt about a colour set's operation from careful examination of the colour -bar test pattern it displays: full details of what to look for are given. UHF AERIAL PREAMP Full details of a simple, self -powered u.h.f. preamplifier for the constructor, using a readily available silicon transistor. Alternative Band IV and V tuning line dimensions are included. CIRCUIT DEVELOPMENTS In a further series of Circuit Notes H. K. Hills takes a look at some of the new circuit techniques that have been introduced with the new breed of single -standard chassis. CONVERGENCE WAVEFORMS In the final instalment of Waveforms in Colour Receivers Gordon J. King describes and illustrates the waveforms required for dynamic convergence, where they are obtained and how used and adjusted, and also looks at pincushion distortion in colour sets. PLUS ALL THE REGULAR FEATURES L ORDER YOUR COPY ON THE FORM BELOW much warmer than usual due to the short-circuited turns acting as the shorted secondary of a down transformer. step- As the d.c. resistance reduction may be only a few ohms, check replacement is the only sure test. Reduced h.t. with normal hum level The cause of this is a low -emission valve rectifier or a high-resitance metal rectifier. Modern silicon power diodes are practically immune from high forward resistance. When any sections on the mains dropper resistor have been changed check that surge limiter replacements are not of excessive value. TO Please reserve/deliver (Name of Newsagent) the JULY issue of PRACTICAL TELEVISION (3/6), on sale JUNE 19th, and continue every month until further notice. NAME ADDRESS 406 SINGLE -STANDARD PART 4 12141NE RECEIVER FOR THE CONSTRUCTOR THIS month we shall deal with the r.f. and i.f. sections of the receiver. The tuner and i.f. strip used were originally intended for dual -standard operation and this includes components which are not necessary for single -standard operation. Both tuner and i.f. strip contain system switches. These should be permanently set to the correct positions as will be described later. The tuner is modified by removal of all the band and system switching mechanics as well as three of the six tuning buttons so that it is mechanically greatly simplified. The i.f. strip is modified to allow the use of sync -tip a.g.c. Sync -tip a.g.c. can only be used where negative vision modulation is employed and since not all readers will be familiar with the technique we shall describe it in detail. Fig. 9 shows the video wave- form present at the output of the detector circuit when negative modulation is employed. Black level corresponds to 77% modulation while the sync pulses extend to 100%. The important point to note is that the sync pulses will always be reaching the 100% level irrespective of the video content of the picture. If, therefore, we can devise a circuit capable of measuring the peak amplitude of the sync pulsesas opposed to the mean -level of the video signal as is generally used on dual -standard sets-the a.g.c. voltage constant for any given signal will be strength, and since the a.g.c. voltage then no longer depends on the video content of the signal the black level will be stable. Thus the circuit will meet the specification requirement laid down, namely that the receiver should provide black level stability. Note that d.c. restoration in the video amplifier stage -previously described-is useless unless the video KEITH CUMMINS a.g.c. is employed-the only way to obtain a stable black level is by the use of a keyed black level clamp. Unfortunately while this provides an improvement in presentation the picture is still not accurately displayed, for although the black level as viewed is constant the ratio of the differing video levels is not. Captions for example, which have a low mean level, are displayed at accentuated contrast against a black background The sync -tip a.g.c. system has the advantage that keyed black level clamping is not required. The simpler process of d.c. restoration is sufficient to enable a completely accurate video presentation to be achieved. The i.f. strip as supplied contains two video detector circuits using two diodes fed from the same end of the final i.f. transformer secondary winding. The diodes are connected in opposite polarity so that both positive- and negative -going outputs are available. The negative -going signal from D2 (Fig. 13) is filtered and taken directly to the video amplifier. The signal lead cannot be screened since the capacitance introduced would attenuate the higher video frequencies. To ensure that stray signals are not induced in the lead and that the capacitance is kept to a minimum the lead is taken individually from the i.f. strip to the video amplifier and kept away from chassis and other conductors as far as possible. The positive -going output diode (Dl, Fig. 13) serves two purposes. It provides both the inter - carrier sound signal. (which is taken off via a 6MHz tuned circuit) and the input to the sync -tip a.g.c. circuit. It is important that the loading of the a.g.c. circuit should be kept not only low but linear output from the detector is stable. The converse of course also applies. so that excessive downward modulation of the inter carrier signal does not occur. Figure 10 shows the basic vision detector system. unstable black level-for example when mean -level technique If the video output from the detector has an Video n 100% sync (which need not always be a separate diode from the set at the transmitter and is not affected by the frequency of the receiver's local oscillator. Thus the frequency of the signal applied to the discriminator or ratio detector used for sound demodulation is Epk (constant) constant. The 6MHz signal for the intercarrier diode is Earth reference be 10% white II- 77% black il should video detector one) to form the 6MHz signal. The frequency of 6MHz is determined by the frequency difference between the sound and vision carriers Earth (a) it .1 I. 77% black 100% sync Epk (constant) reference of intercarrier sound explained that the sound and vision carriers are made to beat together in the intercarrier detector n detector For the benefit of readers not familiar with the (b) Fig. 9: Illustrating how measurement of sync tip amplitude can produce a stable a.g.c. reference level independent of picture content-(a) peak white line, (b) black line. filtered out by the tuned circuit L, C shown. The output from the winding coupled to the tuned circuit is a 6MHz signal modulated in amplitude by the vision signal and in frequency by the sound 407 Video detector To filter and video amp Final IF coil To AGC circuit Intercarrier and AGC detector 10: Vision detector arrangements for video, a.g.c. and intercarrier sound. signal. A limiter stage removes the amplitude modulation to leave the f.m. sound signal. This is fed to a discriminator stage which provides the audio drive to the a.f. amplifier stages. The use of intercarrier sound is particularly advantageous when receiving oscillator u.h.f. transmissions otherwise since drift-which has negligible effect on vision-could adversely affect sound reception. Trl. Under no -signal conditions Trl-an emitterfollower-is turned fully on so that its emitter is neat earth. The video signal as it increases progressively turns Trl off so that its emitter voltage rises positively with respect to earth. The greater the signal the more Trl is turned off. This arrangement is ideal since the 'loading on the detector circuit is greatly reduced. As Trl emitter moves positively so does the base of Tr2 to which it is connected. Base current flows in Tr2, supplied from the 12V rail and not the detector circuit. Tr2 is connected as an The emitter load R31 is shunted by capacitor C20. The pulses of current, npn emitter -follower. in Tr2-which correspond the sync to pulses-charge C20 to a level slightly below that of the sync pulse tips. Because of the long time -constant of C20 and R31 the level is maintained constant during the period Thus it will be seen that 'the voltage developed across C20 is directly proportional to the sync -tip amplitude and hence is a measure of signal between pulses. 230V 12V Contrast VR7 2M C21 Tr2 AC127 AGC detector less in value than R32. Therefore any change in voltage at the junction of R28 and R32 will have little effect on the voltage present at the emitter of Tr2. Similarly R32 is much less in value than R28 so that small changes in voltage applied through R32 will have much greater effect than a similar voltage change applied through R28. The choice, of values has been made so that the setting of the contrast control can set the working point of the a.g.c. amplifier while changes in the sync -tip voltage can be communicated to the amplifier with minimum attenuation. Two other components need mentioning, D13 and D13 is a catching diode which prevents the base of the AC128 a.g.c. amplifier transistor being The tuned circuit shown in Fig. 10 is connected to the resistor R which increases the impedance placed across L, C and also provides a d.c. load resistor across which the a.g.c. sample voltage is developed. It will be seen that the input impedance of the a.g.c. circuit should be high by comparison with R so that both maximum a.g.c. sample voltage is developed and the parallel impedance across L, C kept high. Figure 11 shows in detail the sync -tip a.g.c. circuit. The positive -going video signal is applied via the intercarrier sound take -off circuit to the base of Input from intercarrier and The sync -tip voltage is fed via R32 to the base of the a.g.c. amplifier transistor Tr4 (Fig. 13) type AC128 situated on the i.f. panel. Also fed to this C21. AGC Action flowing a.g.c. circuit. base is a positive bias from VR7, the contrast control (Fig. 11), via R28. It will be seen that R31 is much 6 MHz sound Fig. strength irrespective of picture content. This sync tip voltage is used to control the remainder of the 1 0.11min 013 OA 200 R28 2.2M R29 1M R32 100k To base of AC128 10k AGC amplifier Fig. 11: Sync -tip a.g.c. circuit. driven positive with respect to its emitter which is fed from the 12V rail. C21 provides base -to -emitter a.c. decoupling for the AC128 to enable an unscreened lead to be used to connect the circuit of Fig. 11which is built on the main chassis-to the i.f. panel. In order to connect the circuit of Fig. 11, it is necessary to modify the i.f. panel as shown in Fig. 15 which also gives details of other connections and the functions of relevant tuned circuits. Figure 13 shows the circuit diagram of the i.f. strip in its modified form. Note that the system switch is in the 625 position. All connections to and from the i.f. panel are clearly shown including those to the sync -tip a.g.c. circuit. It will be seen that the AC128 transistor Tr4 referred to earlier acts as an a.g.p. amplifier which is turned off progressively as the signal strength increases. The collector of the AC128 is connected via R18 750n to the emitter of the first vision i.f. amplifier Trl. R18 with the 15051 emitter resistor R4 forms a voltage divider. Under no -signal conditions the BF164 transistor Trl is passing optimum current (i.e. low current) for maximum gain. As the signal increases the a.g.c. system reduces the current in the AC128 amplifier stage so that the emitter to earth potential of the BF164 is reduced. Since the base network tends to hold the base potential constant this is effectively equivalent to increased drive to the BF164 and the current through the BF164 increases. The transistor is designed so that this forward a.g.c. action reduces the gain. Forward a.g.c. has the advantage that cross -modulation of signals is far less than in conventional reverse a.g.c. controlled amplifiers. This becomes obvious when one considers a reverse a.g.c. biased transistor nearly cut off and presented with a large signal: since the 'transistor is under these conditions biased towards the non-linear part of its characteristic intermodulation of signals becomes very likely. The a.g.c. amplifier also provides a control bias for the r.f. amplifier in the tuner. This AF186 pnp transistor is also forward controlled but .the action is delayed so that no reduction of signal occurs in the r.f. stage for weak to medium -strength signals. This precaution ensures that the signal-to-noise ratio is maintained at its best. The basic circuit of the r.f. amplifier a.g.c. arrange- 408 VR6 VR5 R381. R37 Twisted leads R40 R39 n Varistor R34 I 18 b Connec- a tions to field Connections to tine transformer (Hole A) trans- former E = tag earthed to chassis Valve bases: pin 1 only indicated. Numbering is clockwise CONNECTIONS ABOVE CHASSIS (VIA HOLES A -F) Hole A a b c Boost voltage from T3 tag 1 To T3 tag 3 Pulse feed from T3 tag 7 to R38 Hole B a b c d H.T. to T2 primary To T2 secondary from junction C16, C22 To T2 primary from V3 pin 6 T2 secondary earth return ment is shown in Fig. 16. Under no -signal conditions the base-which is fed from the a.g.c. line-is positive by about 10V. The collector to earth resistance is negligible. The emitter is fed from the h.t.+ rail via a 1001a2 resistor (R27 in the main circuit diagram Fig. 4). Being of such a high value this resistor produces heavy d.c. feedback in the emitter circuit of the r.f. amplifier so that changes in the a.g.c. rail voltage initially have very little effect on the current through this transistor. Thus these changes-which control the first i.f. amplifier-do not Hole C H.T. line to R2 and R3 (mounted on top of chassis) a c From R3 From R2 d Feed to c.r.t. first anode (pin 3) b e, f C.R.T. heater feed (pins 1, 8) g Earth return from C23 wired on tube base socket Hole D To c.r.t. pin 2 (grid) a b To c.r.t. pin 7 (cathode) affect the r.f. amplification. This situation continues until the a.g.c. rail falls from its initial level to around +5V. The emitter of the AF186 follows without significant change in current until the catching diode (D12 on the main circuit), whose anode is tied to a supply of approximately +5V, conducts. The emitter is now "caught" at +5V and cannot easily fall farther. If the a.g.c. line now moves towards earth the AF186 passes more current which reduces its gain. Thus the catching voltage sets the point at which 409 To AGC tagstrip via hole E VR7 240V\20 CRT heater supply OV 75V 41 supply R58 :R63 :14 16V supply >upply to CRT oomed & twisted c Tr3 Transformer T1 inter -winding shield connection HT + to VR8 via hole E Hole D Blue T1 primary coirlain and 240V secondary E! --b c d (Hole F) SKT1 HT+ R8 Yellow //Mains lead 2: Complete underchassis wiring details for the 625 -line single -standard receiver. See be/ow for interconnections to top of deck. Hole E To T4C and loudspeaker b To VR9 (volume control) slider (screened lead) To T4D and loudspeaker d 75V feed from T1 (mauve lead) to PCL84 heater (pin 4) e 75V feed from T1 (mauve lead) to PY800 heater (pin 5) To i.f. panel, AC128 base To i.f. panel, junction R12, T4/C28 g h, j 6.3V feed from T1 (white leads) to c.r.t. heater via a k To VR8 slider (brilliance control) m, n Green leads from T1, 16V supply o H.T. feed to T4 (1) p Input from i.f. panel to C6 q 12V feed to tagstrip under tuner, R24 etc. r To T4 (3) from V8 pin 6 I Hole F a b c d hole C e Input to tuner unit f the a.g.c. action commences in the r.f. amplifier. In the main circuit diagram Fig. 4 (published in the April issue) R24 and R25 divide the + 12V supply down to approximately 5V which is applied to the anode of D12 the catching diode. Tuner Modifications We shall next consider the tuner. This is a basic unit capable of tuning over Bands I, III, IV and V. Our use is only for Bands IV and V so the switching Grey lead from T1 to F2 Red lead from T1 to chassis (intershield connection) Grey lead from T1 to junction D3, D4 Brown lead from T1 (240V tap) Yellow lead from T1 (200V tap) Orange lead from T1 to mains neutral mechanism concerned with the other Bands can be removed leaving the bandswitch in the u.h.f. position. Similarly the mechanism associated with system switching can also be removed. To start with the carrier for the system switch slider mechanism can be removed. Only two screws are employed for this purpose. When the mechanism is removed the push -bar for the tuner is revealed. The following sequence should now be followed: (1) Remove slide -switch operating mechanism. This is assisted by removing the band -setting screws. 9 9 47p C5 L3 AGC to tuner 47 C10 AGC circuit R17 2nd amplifier 750 R18 2.2 R6 47 /7777 1 5.) f`-1, ..tt1:1 2.7k R21 P,./77, 1590 1k R9 01 C20 1st Sound IF amp I47p R22 470 R25L R23 P 4.7p C25 T3 2nd Sound IF amp DI AGC and AA119 L ;12 16 10k R34 03 28 C53 R29, 1 R00 2 Out to sync tip r cue ?() 52b L AGC circuit ci 27k R12 R13 2.7k L13 12V In from q 2k R32 (remove) R14 L12 4.7p C29 T 4700p A219 04 AA119 Ratio detector 'rump c49 100 R30 22p C43A ' 270p C 28 Harmonic trap 47 C 27 LI1 A > To AM detector (not used) s't intercarrter detector R271C45 C43 270 Video D2 OA90 detector k 470 7042 1.047i2.7.C42 C38 180 C38A R11 .01 IF amp 3rd Vision RI1A 47 10p C23 C24 10 C22A BFI59 C22 RIO 150 5.6k R8 L8 WS '033 C56 Audio out Link Video out 12V the line output transformer and scan coil assembly at £8 10s. 10d. and a kit of resistors, potentiometers, capacitors, semiconductors and the miscellaneous parts (except for the Varistor) plus T4 at £15 14s. 4d. BF178, a suitable substitute for the BSX21, is supplied for Tr3. Vale Electronics Ltd. can supply a set of valves for £4 4s. 4d., the c.r.t. at £11 19s. plus 10s. 6d. carriage, The component value list (March issue) contained two errors: VR9 is 500 kQ and C43 0.0047 t/F (circuit Fig. 4 shows the correct values). V9 (c.r.t) grid is pin 2. C1 D in Fig. 4 should be shown coded blue, not plain (the plain tag on the specified component is the earth connection). Supplies: Fresh i.f. strip supplies are being sought. For those who cannot a surplus strip, a design for the constructor is being built by the author and will be published Willow IMPORTANT : 012 To emitter circuit of AF186 amp in tuner hole E CI 9 control Earth to brilliance bottom screw securing (F strip to tuner tuner unit. Fig. 14: Connections to the tagstrip mounted under the 230V to VR7 via hole E 12V to tuner 12V to IF panel Fig. 13: Circuit diagram (625 -line sections only) of the printed -circuit i.f. strip used on the prototype, with modifications incorporated as described in the text of this instalment. Component reference numbers above as printed on board. Earthed under the positions of VR6 and R54/R55 are transposed in Fig. 12 compared to the main circuit Fig. 4. I, 110331 C37 H; P 7 'C40 C354 miN270, I 330T01 TS -0 .01 C15 2R7 .01 C1911. Vision IF amp 1st Vision BF164 IF amp T1 AGC1.4. Tr4 ACI2 150 R4 .01 C13 560 R2 BF164 Tr21 33p 33p Ti C17 Underchassis Layout: Note that for ease of wiring Switching is shown in '62V position. C9 15p T 5p To sync tip Le 15p C6 Note:- Circuits dotted thus relate to 405 line receptiOn only and are not used in this application. /7777 l00 Cl 0 IF in C16 3.3p C12 12k R5 L6 O 411 Remove 0C45 transistor AGC out to tuner Remove 2.2k resistor. also 64 NF capacitor connected in parallel Remove AOC signal from capacitor resistor connected here Video out Remove 47k resistor, Remove 5.6k 10pF R28, C21, etc Sound out to VR9 Note: Coils marked (8) are not used in 625 line application Sound ratio detector transformer Earth to Vision and wire lead from chassis IFT (3) Tr1(0C45) base to 12V input this point Intercarrier sound Harmonic take off coil trap to C6 Sound IFT (2) Vision IFT(2) Pass band Sound IFT (1) shaping Vision IF input coil Park system switch slider in this position IFT (1) Fig. 15: The i.f. panel viewed from the foil side, indicating modifications required. Coils marked with a cross in a circle are not used on 625 lines. (2) Remove stop -bar (with pointed ends) situated under the turned -up ends of the push-button rods. (3) Remove the tuner from the push-button assembly by removing three securing screws, two at rear of unit and one under the tagstrip adjacent to the buttons. Hold the gang rotor drive against the spring action and mark the gearwheel and rack with a pencil so that they can be correctly phased when reassembling later. (4) Remove screw in the centre of the mechanism The tuner and i.f. strip can now be fitted together. will be found that the i.f. strip will bolt conveniently to the back of the tuner. Wiring between the two units should be completed according to the main receiver circuit diagram (Fig. 4) with reference also to the printed panel diagram (Fig. 15) and Fig. 14 which shows the layout of the components mounted on a tagstrip attached to the lower side of the tuner. It plate, revealed by removal of the tuner unit. Assembly Above Chassis buttons, adjacent to the push -rod. Pull plastic covers free of buttons. The tuner is finally bolted on to the front panel above the loudspeaker using 2BA nuts, bolts and large washers. The securing bolts pass through two (5) Remove the guide plate at the rear of the (6) With the buttons at the right-hand side and with the mechanism facing you remove the top and bottom two buttons after taking off the circlips holding the plastic centre fine-tuning knobs. Note that the straight tabs may need to be bent slightly to facilitate the final removal. (7) Reassemble the mechanism and tuner so that a simple three -button mechanism remains-without band or system switching facilities. (8) Park the bandswitch slider so that it is fully pushed into the tuner. 230V 100k Catching diode 5V AGC-- - AF186 RF amp Fig. 16: A.G.C. delay system for r.f. stage. holes originally occupied by push -buttons. The large washers fit on the tuner side to cover these holes. The brilliance and volume controls are fitted below the tuner, the brilliance on the left and the volume control to the right as viewed from the front of the receiver. Screened audio cable links the audio output from the i.f. strip to the volume control and then from the volume control to the audio amplifier section of the main chassis. The brightness control is earthed to the tagstrip attached to the tuner unit. All the interconnections can be completed including the aerial coaxial lead which runs from SKT1 to the input point on the tuner halfway along its edge. The other input point is for v.h.f. reception and is not used. The video lead from the i.f. strip to the video stage on the main chassis should be kept away from other leads as far as possible, -continued bn page 424 412 ICONOS iNDIREATH THE DIPOLE ratio is the ratio of width to 'height of a picture projected on to the screen of a cinema, ASPECT printed on film or reproduced on a television receiver screen. (It also concerns the shape of the proscenium opening of a live theatre, the frame of an oil painting and the dimensions of a picture postcard.) So far as television is concerned we are interested in obtaining the maximum information within an acceptable shape. J. L. Baird opted for a square picture with his first crude mechanical contrivance. Later when competing with EMI in Britain and with various other companies in America the ratio of 5 width to 4 height (1.25 to 1) was adopted for a But the ready-made programme material that had been originally filmed for the cinema then had an aspect ratio of 4: 3 or 1.33 to and this was a world standard for both 35mm. and 16mm. film. It stayed that way for years, from the earliest films of Edison, Robert Paul and Lumiere. Thirty years or so later in 1928 sound -tracks time. 1 used up about 100 mil of the picture width and this resulted in an almost square -looking, ugly picture. The American Academy of Motion Picture Arts and Sciences promptly restored the shape to an aspect ratio of 1.33: 1 by reducing the height of the picture and thickening up the frame line. So it remained for many years-until the great gimmick of CinemaScope was introduced by the Fox Film Corporation in America whose technical scouts discovered the anamorphic lens in France. This squeezed the width of the picture within the 35mm. film dimensions and projected it in expanded form to an aspect ratio of 2.55 to 1. This large Fig. 1: Aspect ratios for cinema and television. The effect of using a hard -mask in the camera when the film is shown on television is illustrated on the left. In cinemas the black frame -shape is mechanically adjusted to fit the picture but on television the black topping and tailing bar is very noticeable and most unimpressive to say the least. It has been said by film people that the public is reticent to enter a cinema not presenting a Cinema Scope or Todd A -O picture in the correct manner. It has also been said that TV viewers will probably turn over to another channel when a castrated letterbox shaped picture appears on their sets. The ITV do not like to present CinemaScope-shaped pictures in which the characters at the extreme left and extreme right of picture are heard but not seen. When remedied by some magical electronic " panning" device the viewpoint lurches from side to side. Apart from this subterfuge being crude and mechanical the atmosphere of a scene is probably lost on TV even if it was excellent in a cinema. What is the answer to it all? The Film Production Association put forward recommendations which have been used for years by most British producers without complaint (see Fig. 2). The association found that the most commonly used screen aspect ratios in UK cinemas for nonanamorphic wide-screen films are between to 1.60 and 1 to 1.75 and that from a sample survey of 32 films the majority of feature film producers in the UK shoot films with camera apertures masked to a ratio of either 1 to 1.62 or 1 to 1.52 and com1 pose the picture thereon to a ratio of to 1.85. Television stations however both in the UK and abroad require a picture image area of 0.868 x 1 0.631in. (1 to 1.376) for the satisfactory presentation of films on domestic television receivers, while reduction printing of 35mm. film to 16mm. and wide picture made a big impresion in cinemas, especially when stereo sound was added with four magnetic sound tracks. It still pleases audiences, but the magnetic sound on release prints has goneand the ratio has been reduced to 2.35 to 1. LI r>` 'C"°65 WIDE SCREENS 0-827"x 0-473" jected the top of which could not be seen by the last few (favourite) rows of the cinema stalls because many cinemas too) with a black bar above and below the picture (see Fig. 1). / Projected area Large pictures on large cinema screens were now demanded and non-CinemaScope pictures were pro- the front of the circle obstructed the top line -of sight. So pictures were composed in the camera to aspect ratios of _anything from 1.66 to 1.75 to 1.85 and even to 2 to 1. Sometimes this ratio was shot using a hard -mask in the camera to limit the picture area to that unyielding shape. Films photographed in this way appear on television (and in \\\\\ \ El I Exposed area 0.868"x 9-01", 11-1 Fig. 2: Wide-screen photography for cinema and TV. Above, 35mm. film frame. Inner area for cinema projection, outer area for TV telecine play-off. 413 Special directional screen 8mm. also requires a picture -frame ratio of approxito F376. They also found that the existence of frame -lines wider than normal-due to the introduction of mately 1 Staircase \ masks in camera apertures-can increase the possibility of sound interference in prints with optical sound -tracks. \ Foreground setting Shadow (not seen / by camera)/ Thus the Association while appreciating that pro- ducers wish to compose their films at any aspect ratio which they feel artistically necessary recommended adherence to British Standard 2784: 1956 as being in the industry's best interests. This was Lamp to enable film-makers to obtain the largest possible revenue from their films from all fields of exhibition and to improve the confused situation arising from the varying aspect ratios in use. Actors \---projected4.. TECHNISCOPE Lamp Technicolor are usually well to the fore in meet- the technological problems of Cinerama, CinemaScope, wide-screen and 16mm. film in ing colour and have adapted their " Techniscope" system for television use. Techniscope is an ingenious use of 35mm. film in which the film is pulled down in V